GIFT OF
MICHAEL REESE
PEINCIPLES OF CHEMISTKY
VOL. I.
PRINTED BY
SPOTTISWOODE AXD CO., NEW-STREET SQUARE
LONDON
THE
PRINCIPLES OF CHEMISTRY
BY
D. MENDELEEFF
TRANSLATED FROM THE RUSSIAN (FIFTH EDITION) BY
GEOEGE KAMENSKY, A.E.S.M.
OF T.HB IMPERIAL MINT, ST PETERSBURU
EDITED BY
A. J. GREENAWAY, F.LC.
SCB-EUITUll OF THE JOURNAL OK TilE CHEMICAL SOCIETY
IN TWO VOLUMES
VOL, L
LONDON
LONGMANS, GEEEN, AND CO.
AND NEW YOBK: 15 EAST 16ih STKEET
1891
A II >•>'/?! /> reti n • <l
3°
o
PBEFACE
TO THE
ENGLISH TRANSLATION
IN presenting to the scientific world an English translation of
the text-book of Chemistry written by the great master of the
Periodic Law, we feel that no apology is necessary, for it was in
preparing the first edition of this book that the author was led to
those considerations which resulted in the discovery of that law,
and, moreover, the book is quite unique in its treatment of its
subject.
In order to convey as nearly and clearly as possible the exact
meaning of the author, it has been our endeavour to give, as far as
the genius of the two languages permits, a literal rendering of the
original work. Some exception may no doubt be taken to some of
the sentences, but it was felt on the whole that it would be better
to have some inelegance of language rather than to risk the loss
of the exact shade of meaning that the author had intended to
convey.
We have not considered ourselves at liberty to make any
alterations in the matter of the work, save the omission of two
notes referring to the meaning of Russian words, and of some
details referring to the waters of the streams near St. Petersburg,
which required local knowledge to be of any utility. It has,
however, been necessary to make a considerable change in the
VI PRINCIPLES OF CHEMISTRY
illustrations, as electro-types of the figures in the original could
not be obtained.
Since the publication of the Russian fifth edition, Professor
Mendeleeff has issued some appendices to the work, which will be
found printed at the end of Volume II. We have to express our
thanks to the Managers of the Royal Institution for permission to
reprint the lecture delivered at the Royal Institution by Professor
Mendeleeff (Appendix I.), and to the Council of the Chemical
Society for permission to reprint the Faraday lecture which forms
Appendix II.
In conclusion, we would express our gratitude to Professor
Kinch for the aid so kindly given in revising the sheets for
the press.
G. K.
A. J. G.
October, 1891.
AUTHOR'S PREFACE
TO
THE FIFTH EDITION
THIS work was written during the years 1868-1870, its object
being to acquaint the student not only with the methods of ob-
servation, the experimental facts, and the laws of chemistry, but
also with the aspect of this science towards the invariable sub-
stance of varying matter. If the facts themselves include the
person who observes them, then how much more inevitable is the
reflection of personality in giving an account of methods and of
philosophical speculations ? For the same reason there will inevi-
tably be much that is subjective in every objective exposition of
science. And as an individual production is only significant in
virtue of that which has preceded and which surrounds it, so it
essentially resembles a mirror which in reflecting exaggerates the
size and clearness of neighbouring objects, and causes a person
near it to see reflected most plainly those objects which are 011 the
side to which it is directed. Although I have endeavoured to make
my book a true mirror directed towards the domains of chemical
transformations, yet involuntarily those influences near to me have
been the most clearly reflected, the most brightly illuminated,
and have tinted the entire work with their colouring. In this
way the chief peculiarity of the book has been determined. Ex-
perimental and practical data occupy their place, but the philo-
Vlll PRINCIPLES OF CHEMISTRY
sophical principles of our science form the chief theme of the work.
In former times sciences, like bridges, could only be built up by
supporting them on a few deep abutments and long girders. In
addition to the exposition of the principles of chemistry, it has
been my desire to show how science has now been built up like
a suspension bridge, supported by the united strength of a number
of slender, but firmly-fixed, threads, which individually are of
little strength, and has thus been carried over difficulties which
before appeared impassable. In comparing the science of the past,
the present, and the future, in placing the particulars of its re-
stricted experiments side by side with its aspirations for unbounded
and infinite truth, and in restraining myself from yielding to a bias
towards following the most attractive representation, I have en-
deavoured to incite in the reader a spirit of inquiry, which, unsatis-
fied with speculative reasonings alone, should subject every idea
to experiment, excite the habit of stubborn woi-k, necessitate a
knowledge of the past, and a search for fresh threads to complete
the bridge over the bottomless unknown. Experience proves that
it is possible by this means to avoid two equally pernicious extremes,
the Utopian — a visionary contemplation which proceeds from a
current of thought only — and the realistic stagnation which is
content with bare facts. In sciences like chemistry, which treat
of ideas as well as of the substances of nature, experience demon-
strates at every step that the work of the past has availed much,
and that without it it would be impossible to advance ' into the
ocean of the unknown/ We are compelled to value their history,
to cast aside classical illusions, and to engage in a work which not
only gives mental satisfaction but is also practically useful.1
1 Chemistry, like every other science, is at once a means and an end. It is a
means of attaining certain practicable aspirations. Thus, by its assistance, the
obtaining of matter in its various forms is facilitated ; it shows new possibilities
of availing ourselves of the forces of nature, indicates the methods of preparing
many substances, points out their properties, etc. In this sense chemistry is
closely connected with the work of the manufacturer and the artisan, its sphere
is active, and is a means of promoting general welfare. Besides this honourable
vocation, chemistry has another. With it, as with every other elaborated science,
there are many lofty aspirations, the contemplation of which serves to inspire its
workers and partisans. This contemplation comprises not only the principal data
I'KKI'ACE ix
Thus the desire to direct those thirsting for truth to the pure
source of the science of the forces acting throughout nature forms
of the science, but also the generally-accepted deductions, and also hypotheses,
which refer to phenomena as yet but imperfectly known. In this latter sense
scientific contemplation varies much with times and persons, it bears the stamp
of creative power, and comprehends the highest branch of scientific progress.
In that pure enjoyment experienced on approaching to the ideal, in that eagerness
to draw aside the veil from the hidden truth, and even in that discord which
exists between the various workers, we ought to see the surest pledges of further
scientific success. Science thus advances, discovering new truths, and at the
same time obtaining practical results. The edifice of science not only requires
material but also a plan, and necessitates the work of preparing the materials,
putting them together, working out the plans and the symmetrical proportions
of the various parts. To conceive, understand, and grasp the whole symmetry of
the scientific edifice, including its unfinished portions, is equivalent to tasting
that enjoyment only conveyed by the highest forms of beauty and truth. Without
the material, the plan alone is but a castle in the air, a mere possibility, whilst
the material without a plan is but useless matter ; all depends on the concordance
of the materials with the plan and execution, and the general harmony thereby
attained, In the work of science, the artisan, architect, and creator are very
often one and the same individual, but sometimes, as in other walks of life,
there is a difference between them ; sometimes the plan is preconceived, some-
times it follows the preparation and accumulation of the raw material. Free
access to the edifice of science is not only allowed to those who devised the plan,
worked out the detailed drawings, prepared the materials, or piled up the brick-
work, but also to all those who are desirous of making a close acquaintance with
the plan, and wish to avoid dwelling in the vaults or in the garrets where the
useless lumber is stored.
Knowing how contented, free, and joyful is life in the realms of science, one
fervently wishes that many would enter their portals. On this account many
pages of this treatise are unwittingly stamped with the earnest desire that the
habits of chemical contemplation which I have endeavoured to instil into the
minds of my readers will incite them to the further study of science. Science
will then flourish in them and by them, on a fuller acquaintance not only with
that little which is enclosed within the narrow limits of my work, but with the
further learning which they must imbibe in order to make themselves masters of
our science and partakers in its further advancement.
Those who enlist in the cause of science have no reason to fear when they
remember the urgent need for practical workers in the spheres of agriculture,
arts, and manufacture. By summoning adherents to the work of theoretical
chemistry, I am confident that I call them to a most useful labour, to the
habit of dealing correctly with nature and its laws, and to the possibility of
becoming truly practical men. In order to become actual chemists, it is
necessary for beginners to be well and closely acquainted with three impor-
tant branches of chemistry— analytical, organic, and theoretical. That part of
chemistry which is dealt with in this treatise is only the ground work of the edifice.
For the learning and development of chemistry in its truest and fullest sense,
be.iri nners ought, in the first place, to turn their attention to the practical work of
analytical chemistry: in the second place, to practical and theoretical urquaiut-
X PRINCIPLES OF CHEMISTRY
the first and most important aim of this book. The time has ar-
rived when a knowledge of physics and chemistry forms as im-
portant a part of education as that of the classics did two centuries
ago. In those days the nations which excelled in classical learning
stood foremost, just as now the most advanced are those which are
superior in the knowledge of the natural sciences. I also wished
to show in an elementary treatise on chemistry the palpable ad-
vantages gained by the application of the periodic law, which I first
saw in its entirety in the year 1869 when I was engaged in writing
the first edition of this book, in which, indeed, the law was first
enunciated. Then, however, this law was not established so firmly
as now, when so many of its consequences have been verified by
the researches of numerous chemists, and especially by Roscoe,
Lecoq de Boisbaudran, Nilson, Brauner, Thorpe, Carnelley, Laurie,
Winkler, and others. As the entire scheme of this work2 is sub-
jected to the law of periodicity, which may be illustrated in a
ance with some special chemical question, studying the original treatises of the
investigators of the subject (at first, under the direction of experienced teachers),
because in working out particular facts the faculty of judgment and of correct
criticism becomes sharpened ; in the third place, to a knowledge of current scien-
tific questions through the special chemical journals and papers, and by inter-
course with other chemists. The time has come to turn aside from visionary
contemplation, from platonic aspirations, and from classical verbosity, and to
enter the regions of actual labour for the common weal, and to prove that the
study of science is not only an excellent education for youth, but that it instils
the virtues of labour and truth, and creates solid national wealth, material and
mental, which without it would be imattainable. Science, which deals with the
infinite, is itself without bounds.
2 I recommend those who are commencing to study chemistry with my book
to first learn only Ballot is printed in the large type, because in that part I have en-
deavoured to concentrate all the fundamental, indispensable knowledge required
for the study of chemistry. In the footnotes, printed in small type (which I advise
being read only after the large text has been mastered), certain details are dis-
cussed ; they are either further examples, or debatable questions on existing ideas
which I thought indispensable to lay before those entering into the sphere of
science, or certain historical and technical details which might be withdrawn
from the fundamental portion of the book. Without intending to attain in my
treatise to the completeness of a work of reference, I have still endeavoured
to express the principal developments of science as they concern the chemical
elements viewed in that aspect in which they appeared to me after long con-
tinued study of the subject and participation in the contemporary advance of
knowledge.
PKEFACE xi
tabular form by placing the elements in series, groups, and
periods, two such tables are given at the end of this preface.
In this fifth edition I have not altered any essential feature of
the original work, but have enlarged it in two directions. First,
the doctrine of chemical equilibria, originally introduced by
Berthollet and Henri Sainte-Claire Deville, is discussed more
fully and minutely than in the earlier editions, as it has during
recent years been established on a much firmer footing; and,
second, the descriptive data referring to the elements have been
increased by many new facts.
D. MENDELEEFF.
Xll
PRINCIPLES OF CHEMISTRY
i <3
5 ' fi
&
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0 . H .
' 1 ' fi '
6 . 1 - H
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be ' fl
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o o
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Xlll
TABLE II.
THE ATOMIC WEIGHTS OF THE ELEMENTS
Distribution of the Elements in Periods
Groups
Higher
Salt-
forming
Oxides
Typical or
1st small
Period
Large Periods
1st
2nd
3rd
4th
5th
I.
R.O
Li = 7
K 39
Rb 85
Cs 133
—
—
II.
RO
Be =9
Ca40
S 87
Bal37
—
—
III.
RA
B =11
Sc 44
Y 89
La 138
Ybl73
—
IV.
R02
C =12
Ti 48
Zr 90
Ce 140
—
Th232
V.
RA
N =14
V 51
Nb 94
—
Ta 182
—
VI.
RO,
O =16
Cr 52
Mo 96
—
W 184
Ur 240
VII.
RA
F =19
Mn 55
—
—
—
—
Fe56
Rul03
—
Os 191
—
VIII.
-
Co 58-5
Rhl04
—
Ir 193
—
Ni59
Pdl06
—
Pt 196
—
I.
R20
H-l.Na=23
Cu63
AglOS
—
Aul98
—
II.
RO
Mg = 2*
Zn 65
Cdll2
—
Hg200
—
III.
RA
Al =27
Ga70
In 113
—
Tl 2C4
—
IV.
R02
Si =28
Ge72
Sn 118
—
Pb206
—
V.
RA
P =31
As 75-
Sb 120
—
Bi 208
—
VI.
RO,
S =32
Se 79
Tel25
—
—
—
VII.
RA
Cl =35-5
Br 80
I 127
—
—
—
2nd small
Period
1st
2nd
3rd
4th
6th
Large Periods
CONTENTS
OF
THE FIRST VOLUME
PAGE
TRANSLATORS' PREFACE ....... v
AUTHOR'S PREFACE TO THE FIFTH EDITION . . . . .. vii
TABLE OF THE DISTRIBUTION OF THE ELEMENTS IN GROUPS AND
SERIES . . . . . . . xii
TABLE OF THE ATOMIC WEIGHTS OF THE ELEMENTS. DISTRIBUTION
OF THE ELEMENTS IN PERIODS ..... xiii
INTRODUCTION . . . 1
CHAPTER
I. ON WATER AND ITS COMPOUNDS . . . . .40
II. THE COMPOSITION OF WATER. HYDROGEN . . . 112
III. OXYGEN AND THE CHIEF ASPECTS OF ITS SALINE COMBINATIONS . 151
IV. OZONE AND HYDROGEN PEROXIDE. DALTON'S LAW . . . 197
V. NITROGEN AND AlR ....... 221
VI. THE COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN . 243
VII. MOLECULES AND ATOMS. THE LAWS OF GAY-LUSSAC AND
AVOGADRO-GERHARDT . . . . . . 292
VIII. CARBON AND THE HYDROCARBONS . ... 3*2(5
IX COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN . . 307
XVI PRINCIPLES Otf CHEMISTRY
< HAl'TKR
X. SODIUM CHLORIDE. BERTHOLLET'S LAWS. HYDROCHLORIC ACID .
THE HALOGENS I CHLORINE, BROMINE, IODINE, AND FLUORINE . 45
XII. SODIUM . . . . . . . . 5CK
XIII. POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIUM. SPECTRUM
ANALYSIS. ....... 535
XIV. THE VALENCY AND SPECIFIC HEAT OF THE METALS. MAGNESIUM,
CALCIUM, STRONTIUM, BARIUM, AND BERYLLIUM . 572
Erratum.
Page 91, line 4 from foot,/cr Prinsep read Pierson.
•
PEINCIPLES OF CHEMISTRY
INTRODUCTION
CHEMISTRY is concerned with the study l of the homogeneous substances
1 The investigation of a substance or phenomenon of nature consists (a) in determin-
ing the relation of the thing under investigation to that which is already known, either
from former studies, or from experiment, or from the consciousness of the common sur-
roundings of life — that is, in determining and expressing the quality of the unknown by
the aid of that which is known; (6) in measuring all that which can be subjected to
measurement, and thereby denoting the quantitative relation of that under investigation
to that already known and its relation to the categories of time, space, temperature,
ma^s. Are. ; (c) in determining the position held by the thing under investigation in the
system of the things known, guided by both qualitative and quantitative data; (d) in
finding, from the quantities which have been measured, the empirical (visible) depen-
dence (function, or ' law,' as it is sometimes termed) of variable factors — for instance, the
dependence of the composition of the substance on its properties, of temperature on
time, of time on locality, &c. ; (r] in framing hypotheses or propositions as to the actual
cause and true nature of the relation between that studied (measured or observed) and
that which is known or the categories of time, space, Arc.; (f) in verifying the logical
consequences of the hypotheses by experiment ; and (g) in advancing a theory which
shall account for the nature of the properties of that studied in its relations with things
already known and with those conditions or categories among which it exists. It is
certain that it is only possible to thus study, when we have taken as a basis some incon-
testable fact which is self-evident to our understanding ; as, for instance, number, time,
space, movement, or mass. The determination o-f such primary or fundamental concep-
tions (categories), although not excluded from the possibility of investigation, frequently
does not subject itself to our present mode of scientific generalisation. Hence it follows
in the investigation of anything, there always remains something which is recognised
without investigation, or admitted as a known factor. The axioms of geometry may be
taken as an example. Thus in the science of biology it is necessary to admit the faculty
of organisms for multiplying themselves, as a conception whose meaning is yet unknown.
Thus in the study of chemistry the notion of elements must be recognised without
hardly any further analysis. However, by first investigating that which is visible and
subject to direct observation by the organs of the senses, we may hope that, first,
hypotheses will be arrived at, and afterwards theories of that which has now to be placed
at the basis of our investigations. The minds of the ancients strove to at once seize the
very fundamental categories of investigation, whilst all the successes of recent know-
ledge are based on the above-cited method of investigation without the determination of
' the beginning of all beginnings.' By following this inductive method, the exact sciences
VOL. I. B
2 PRINCIPLES OF CHEMISTRY
or material 2 of which all the objects of the universe are made up, with
the transformations of these substances into each other, and with the
phenomena 3 which accompany such transformations. Every chemical
have already succeeded in becoming acquainted with certainty with much of the invi-
sible world, which directly is imperceptible to the organs of sense (for example, the mole-
cular movement of all bodies, the composition of the heavenly luminaries, the paths of
their movement, the necessity for the existence of substances which cannot be subjected
to experiment, &c.), and have verified the knowledge thus obtained, and employed it for
increasing the interests of human life ; and therefore it may be safely said that the induc-
tive method of investigation is a more perfect mode of acquiring knowledge than the
deductive method alone (starting from a little of the -unknown accepted as incontestable
to arrive at the much which is visible and observable) by which the ancients strove to
embrace the universe. By investigating the universe by an inductive method (endeavour-
ing from the much which is observable to arrive at a little which may be verified and
is indubitable) the new science refuses to recognise dogma as truth, but through reason,
by a slow and laborious method of investigation, strives for and attains to true de-
ductions.
2 A substance or material is that which occupies space and has weight. That is,
which presents a mass which is attracted by the earth and by other masses of material,
and of which the objects of nature are composed, and through which the movements and
phenomena of nature are accomplished. It is easy to find out by examining and
investigating, by various methods, the objects met with in nature and in the arts, that
some of them are homogeneous, whilst others are composed of a mixture of several
homogeneous substances. This is most clearly seen in solid substances. The metals
used in the arts (for example, gold, iron, copper) should be distinguished for their
homogeneity, otherwise they are brittle and unfit for many uses. Homogeneous matter
exhibits similar properties in all its parts. By breaking up a homogeneous substance we
obtain parts which, although different in form, resemble each other in their properties.
Glass, the best qualities of sugar, marble, &c., are examples of homogeneous substances.
But examples of non-homogeneous substances ai'e much more frequent in nature and the
arts. Thus the majority of the rocks are not homogeneous. In porphyries bright pieces
of a mineral called ' orthoclase ' are often seen strewn amongst the dark mass of the rock.
In ordinary red granite it is easy to distinguish large pieces of orthoclase mixed with
dark semi-transparent quartz and flexible laminae of mica. Nor are plants and animals
homogeneous. Thus leaves are composed of a skin, fibre, pulp, sap, and a green colouring
matter. This is clearly seen by examining under a microscope a thin slice cut off a leaf.
As an example of those non-homogeneous substances which are produced artificially,
gunpowder may be cited, which is prepared by mixing together known proportions of
sulphur, nitre, and charcoal. Many liquids, also, are not homogeneous, as may be observed
by the aid of the microscope, when drops of blood are seen to consist of a colourless
liquid in which red corpuscules, invisible to the naked eye owing to their small size, are
floating about. It is these corpuscules which give blood its peculiar colour. Milk is also
a transparent liquid, in which microscopical drops of fat are floating, and which rise to the
top when milk is left at rest, forming cream. When the fat is beaten up (churned) the
separate drops collect into one mass. It is possible to extract from every non-
homogeneous substance those homogeneous substances of which it is made up. Thus
orthoclase may be separated from porphyry by breaking it off. So also gold is extracted
from gold-bearing sand by washing away the mixture of clay and sand. Chemistry deals
only with the homogeneous substances met with in nature, or extracted from natural or
artificial non-homogeneous substance. The various mixtures found in nature form the
subjects of other natural sciences — as geognosy, botany, zoology, anatomy, &c.
3 All those events which are accomplished by substances in time, are termed ' pheno-
mena.' Phenomena in themselves form the fundamental subject of the study of physics.
Movement is the primary and most generally understood form of phenomenon, and there-
fore we endeavour to reason about other phenomena as clearly as when dealing with move-
iNTHonrrnox 8
change or reaction,4 as it is called, can only take place under a condi-
tion of most intimate and close contact of the reacting substances,5 and
is determined by the forces proper to the smallest invisible particles
(molecules) of matter. We must distinguish three chief classes of
chemical transformations.
1. Combination is a reaction in which the union of two substances
yields a new one, or in general terms, from a given number of sub-
stances a lesser number is produced. Thus, by heating a mixture of
iron and sulphur6 a single new substance is produced, iron sulphide, in
which the constituent substances cannot be distinguished even by the
highest magnifying power. Before the reaction, the iron could be
separated from the mixture by a magnet, and the sulphur by dissolving
it in certain oily liquids ; 7 in general, before combination they might
be mechanically separated from each other, but after combination both
substances penetrate into each other, and are then neither mechanically
separable nor individually distinguishable. As a rule, reactions of
direct combination are accompanied by an evolution of heat, and the
common case of combustion, evolving heat, consists in the combination
of combustible substances with a portion (oxygen) of the atmosphere,
ment. Therefore, mechanics, which treats of movement, forms the fundamental science
of natural philosophy, and all other sciences endeavour to reduce the phenomena with
which they are concerned to mechanical principles. Astronomy was the first to take
to this path of reasoning, and succeeded in many cases in reducing astronomical to
purely mechanical phenomena. Chemistry and physics, physiology and biology are
proceeding in the same direction.
4 The verb ' to react ' means to act or change chemically.
5 If a phenomenon proceeds at visible or measurable distances (as, for instance,
magnetic attraction or gravity) it cannot be ascribed to chemical phenomena, which are
only accomplished at distances immeasurably small and undistinguishable to the eye or
the microscope ; that is to say, which belong to the number of purely molecular pheno-
mena. When a change of material is accomplished within a substance without visible
motion or the interference of foreign matters (for instance, when new wine ' ages ' by
keeping, and acquires a peculiar aroma), it may be classed as a chemical phenomenon ; but
the ordinary cases of chemical reaction are accomplished by the mutual action of different
substances which, previously free, on reaction mutually permeate each other.
f> For this purpose a piece of iron may be made red hot in a smith's furnace, and then
placed in contact with a lump of sulphur, when iron sulphide will be obtained as a
molten liquid, the combination being accompanied by a visible increase in the glow of
the iron. Or else iron filings are mixed with powdered sulphur in the proportion of
5 parts of iron to 3 parts of sulphur, and the mixture placed in a glass tube, which is
then partially heated. Combination does not commence without the aid of external
heat, but when once started in any portion of the mixture it extends throughout the
entire mass, because the portion first heated evolves sufficient heat in forming iron
sulphide to raise the adjacent parts of the mixture to the temperature required for
starting the reaction. The rise in temperature thus obtained is so high as to soften the
glass tube.
7 Sulphur is slightly soluble in many thin oils; it is very soluble in carbon bisulphide
and in some other liquids. Iron is insoluble in carbon bisulphide, and therefore the
sulphur can be dissolved away from the iron.
B 2
4 PWNCIPLES OF CHEMISTRY
the gases and vapours contained in the smoke being the products of
combination.
2. Reactions of decomposition are cases the reverse to those of
combination, that is, in which one substance gives two — or, in general, a
given number of substances a greater number. Thus, by heating wood
(and also coal and many animal or vegetable substances) without access
to air, a combustible gas, a watery liquid, tar, and carbon are obtained.
It is in this way that tar, lighting gas, and charcoal are prepared on a
large scale.8 All limestones, for example, flagstones, chalk, or marble,
are decomposed by heating to redness into lime and a peculiar gas
called carbonic anhydride. A similar decomposition, taking place,
however, at a much lower temperature, proceeds with the green copper
carbonate which enters into the composition of malachite. This ex-
ample will be studied more in detail presently. Whilst heat is evolved
in the ordinary reactions of combination, it is, on the contrary, con-
sumed in the reactions of decomposition.
3. The third class of chemical reactions — where the number of acting
substances is equal to the number of substances formed— consists, as it
were, of an association of decomposition and combination. If, for
instance, two compounds A and B are taken and they react on each
other to form the substances C and D, then supposing that A is de-
composed into D and E, and that E combines with B to form C, we
have a reaction in which two substances A, or D E, and B were taken
and two others C, or E B, and D were produced. Such reactions ought
to be placed under the general term of reactions of 'rearrangement,'
and the particular case where two substances give two fresh ones,
reactions of * substitution.' 9 Thus, if a piece of iron be immersed in a
solution of blue vitriol (copper sulphate), copper is formed — or, rather,
8 Decomposition of this kind is termed ' dry distillation ' because, as in distillation,
the substance is heated and vapours are given off which, on cooling, condense into
liquids. In general, decomposition, in absorbing heat, presents much in common to a
physical change of state — such as, for example, that of a liquid into a gas. Deville
likened complete decomposition to boiling, and compared partial decomposition, when a
portion of a substance is not decomposed in the presence of its products of decomposition
(or dissociation), to evaporation.
9 A reaction of rearrangement may in certain cases take place with one substance
only ; that is to say, a substance may by itself change into a new isomeric form. Thus,
for example, if hard yellow sulphur be heated to a temperature of 250° and then poured
into cold water it gives, on cooling, a soft, brown variety. Ordinary phosphorus, which
is transparent, poisonous, and phosphorescent in the dark (in air), gives, after being
heated at 270° (in an atmosphere incapable of supporting combustion, such as steam), an
opaque, red, and non-poisonous isomeric variety, which is not phosphorescent. Cases of
isomerism point out the possibility of an internal rearrangement in a substance, and are
the result of an alteration in the grouping of the same elements, just as a certain number
of balls may be grouped in figures and forms of different shapes and of various properties.
INTRODUCTION 5
separated out, and green vitriol (iron sulphate, which only differs from
the blue vitriol in that the iron has replaced the copper) is obtained in
solution. In this manner iron may be coated with copper-, so also copper
with silver ; such reactions are frequently made use of in practice.
The majority of the chemical changes accomplished in nature and
the arts are very complicated, as they consist of an association of many
separate and simultaneous combinations, decompositions, and replace-
ments. In this natural complexity of chemical phenomena is discovered
the chief reason why for so many centuries chemistry did not exist as
an exact science ; that is to say, that although many chemical changes
were known and made use of,10 yet their real nature was unknown, nor
could they be foreseen or directed at will. Another reason for the
tardy progress of chemical knowledge is the participation of gaseous
substances, especially air, in many reactions. The true comprehension
of air as a ponderable substance, and of gases in general as peculiar elastic
and dispersive states of matter, was only arrived at in the sixteenth and
seventeenth centuries, and it was only after this that the transformations
of substances could form a science. Up to that time, without under-
standing the invisible and yet ponderable gaseous and vaporous states
of substances, it was impossible to form any fundamental chemical
evidence, because gases escaped from notice between the acting and
resultant substances. It is easy from the impression conveyed to us by
the phenomena we observe to form the opinion that matter is created
and destroyed : a whole mass of trees burn, and there only remains a
little charcoal and ash, whilst from one small seed there grows little
by little a majestic tree. In one case matter seems to be destroyed, and
in the other to be created. This conclusion is arrived at because the
formation or consumption of gases, being under the circumstances
invisible to the eye, is not noted. When wood burns it undergoes a
chemical change into gaseous products, which escape as smoke. A very
simple experiment will prove this. By collecting the smoke it may be
observed that it contains gases which differ entirely from air, being
incapable of supporting combustion or respiration. These gases may
be weighed, and it will then be seen that their weight exceeds that of
the wood taken. This increase in weight arises from the fact that, in
burning, the component parts of the wood combine with a portion of
the air ; in like manner iron increases in weight by rusting. In burn-
ing gunpowder its substance is not destroyed, but only converted into
gases and smoke. So also in the growth of a tree ; the seed does not
10 Thus tin- ancients knew how toconvert the juice of grapes containing the saccharine
principle (glucose) into wine or vinegar, or how to extract metals from the ores which
are found in the earth's crust, and how to prepare glass from earthy substances.
6 PRINCIPLES OF CHEMISTRY
increase in mass of itself and from itself, but it grows because it absorbs
gases from the atmosphere and sucks water and substances dissolved
therein from the earth through its roots. The sap and solid substances
which give plants their form are produced from these absorbed gases
and liquids by complicated chemical processes. The gases and liquids
are converted into solid substances by the plants themselves. Plants
not only do not increase in size, but die, in a gas which does not contain
the constituents of air. When moist substances dry they decrease in
weight ; when water evaporates we know that it does riot disappear,
but will return from the atmosphere as rain, dew, and snow. When
water is absorbed by the earth, it does not disappear there for ever, but
accumulates somewhere underground, from whence it afterwards flows
forth as a spring. Thus matter does not disappear and is not created,
but only undergoes various physical and chemical transformations — that
is to say, changes its locality and form. Matter remains on the earth
in the same quantity as before ; in a word it is, as far as we are con-
cerned, everlasting. It was difficult to submit this simple and primary
truth of chemistry to investigation, but when once made clear it rapidly
spread, and now seems as natural and simple as many truths which
have been acknowledged for ages. Mario tte and other savants of the
seventeenth century already suspected the existence of the law of the
indestructibility of matter, but they made no efforts to express it or to
apply it to the ends of science. The experiments by means of which
this simple law was arrived at were made during the latter half of the
last century by the founder of contemporary chemistry, LAVOISIER, the
French Academician and mayor. The numerous experiments of this
savant were conducted with the aid of the balance, which is the only
means of directly and accurately determining the quantity of matter.
Lavoisier found, by weighing all the substances, and even the
apparatus, used in every experiment, and then weighing the substances
obtained after the chemical change, that the sum of the weights of the
substances formed was always equal to the sum of the weights of the
substances taken ; or, in other words : MATTER is NOT CREATED AND
DOES NOT DISAPPEAR, or that, matter is everlastiny. This expression
naturally includes a hypothesis, but our only aim in using it is to con-
cisely express the following lengthy period — That in all experiments,
and in all the investigated phenomena of nature, it has never been
observed that the weight of the substances formed was less or greater
(as far as accuracy of weighing permits) than the weight of the sub-,
stances originally taken, and as weight is proportional to mass11 or
11 The idea of the mass of matter was first shaped into an exact form by Galileo (died
1642), and more especially by Newton (born 1643, died 1727), in the glorious epoch of the
BPTRODUCnOH 7
quantity of matter, it follows that no one has ever succeeded in observ-
ing a disappearance of matter or its appearance in fresh quantities.
The law of the indestructibility of matter endows all chemical investi-
gations with exactitude, as, on its basis, an equation may be formed for
every chemical reaction. If in any reaction the weights of the sub-
stances taken be designated by the letters A, B, C, &c., and the
weights of the substances formed by the letters M, N, 0, &c., then
A + B + C + = M + N + O +
Therefore, should the weight of one of the acting or resultant sub-
stances be unknown, it may be determined by solving the equation.
The chemist, in applying the law of the indestructibility of matter,
must never lose sight of any one of the acting or resultant substances.
Should such an oversight be made, it will at once be remarked from
the sum of the weights of the substances taken being unequal to the
sum of the weights of the substances formed. All the progress made
by chemistry during the end of the last, and in the present, century is
entirely and immovably founded on the law of the indestructibility of
matter. It is absolutely necessary in beginning the study of chemistry
to become familiar with the simple truth which is expressed by this
law, and for this purpose several examples elucidating its application
will now be cited.
1. It is well known that iron rusts in damp air,1'2 and that when
heated to redness in air it becomes coated with scoria (oxide), having,
like rust, the appearance of an earthy substance resembling some of the
iron ores from which metallic iron is extracted. If the iron is weighed
before and after the formation of the scoria or rust, it will be found
that the metal has increased in weight during the operation.13 It
development of the principles of inductive reasoning enunciated by Bacon and Descartes
in their philosophical treatises. Shortly after the death of Newton, Lavoisier, whose
fame in natural philosophy should rank with that of Galileo and Newton, was born on
August 20, 1743. The death of Lavoisier occurred during the Reign of Terror of the
French Revolution, when he, together with twenty-six other chief farmers of the revenue,
was guillotined on May 8, 1794, at Paris, but his works and thoughts have made him
immortal.
12 By covering iron with an enamel, or varnish, or with unrustable metals (such as
nickel), or a coating of paraffin, or other similar substances, it is protected from the air
ami moisture, and so kept from rusting.
1 • Such an experiment may easily be made by taking the finest (unrusted) iron filings
(ordinary tilings must be first washed in ether, dried, and passed through a very fine
sieve). The filings thus obtained are capable of burning directly in air (by oxidising or
forming rust), especially when they hang (are attracted) on a magnet. A compact piece
of iron does not burn in air, but spongy iron glows and smoulders like tinder. In
making the experiment, a horse-shoe magnet is fixed, with the poles downwards, on one
arm of a rather sensitive balance, and the iron filings are applied to the magnet (on a
8 PRINCIPLES OF CHEMISTRY
can easily be proved that this increase in weight and formation of
earthy substances from the metal is accomplished at the expense of
the atmosphere, and mainly, as Lavoisier proved, at the expense of
that portion which is called oxygen, and, as will afterwards be
explained, supports combustion. In fact, in a vacuum, or in gases
which do not contain oxygen, for instance, in hydrogen or nitrogen,
the iron neither rusts nor becomes coated with scoria. Had the iron
not been weighed, the participation of the oxygen of the atmosphere in
its transformation into an earthy substance might have easily passed
unnoticed, as was formerly the case, when phenomena like the
above were, for this reason, misunderstood. It is evident from the
law of the indestructibility of matter that as the iron increases in
weight in its conversion into rust, the latter must be a more complex
substance than the iron itself, and its formation is due to a reaction of
combination. Were not this chemical change studied in regard to
mass, and did we not know of the ponderability of air, and of its
capacity to take part in the phenomena of combustion, we might form
an entirely wrong opinion about it, and might, for instance, consider
rust to be a simpler substance than iron, and explain the formation of
rust as the removal of something from the iron. Such, indeed, was
the general opinion prior to Lavoisier, when it was held that iron con-
tained a certain unknown substance called * phlogiston,' and that rust
was iron deprived of this supposed substance.
2. Copper carbonate (in the form of a powder, or as the well-known
green mineral called ' malachite,' which is used for making ornaments,
or as an ore for the extraction of copper) changes into a black sub-
stance called 'copper oxide' when heated to redness.14 This black
sheet of paper) so as to form a beard about the poles. The balance pan should be exactly
under the filings on the magnet, in order that any which might fall from it should not
alter the weight. The filings, having been weighed, are set light to by applying the flame
of a candle; they easily take fire, and go on burning by themselves, forming rust.
When the combustion is ended, it will be clear that the iron has increased in weight ;
from 5£ parts by weight of iron filings taken, there are obtained, by complete com-
bustion, 7£ parts by weight of rust. Consequently, if about 5 grams of filings be
applied to the magnet, the increase in weight will be clearly seen by the weights that are
required to restore equilibrium. This experiment proceeds so easily and quickly that it
may be conveniently demonstrated, as a proof of the increase of weight at the expense of
air and of its transformation into the solid iron-rust.
14 For the purpose of experiment, it is most convenient to take copper carbonate, pre-
pared by the experimenter himself, by adding a solution of sodium carbonate to a solution
of copper sulphate. The precipitate (deposit) so formed is collected on a filter, washed,
and dried. The decomposition of copper carbonate into copper oxide is effected by so
moderate a heat that it may be accompished in a glass vessel heated by a lamp. For
this purpose a thin glass tube, closed at one end, and called a ' test tube," may be em-
ployed, or else a vessel called a ' retort.' The experiment is carried on, as described in the
third example above, by collecting the carbonic anhydride over a water bath, as will be
afterwards explained.
UJTBODUCWON (J
substance is also obtain* 'd by heating copper to redness in air — that is,
it is the scoria or oxidation product of copper. The weight of the
black oxide of copper left is less than that of the copper carbonate
originally taken, and therefore we consider the reaction which occurred
to have been one of decomposition, and that by it something was sepa-
rated from the green copper carbonate, and in fact by closing the orifice
of the vessel in which the copper carbonate is heated with a well-
litting cork, through which a gas delivery tube15 passes whose end is
immersed under water, it will be observed that on heating, a gas is
formed which bubbles through the water. This gas can be easily
collected, as will presently be described, and it will be found to essen-
tially differ from air in many respects ; for instance, a burning taper
is extinguished in it as if it had been plunged into water. If weighing
had not proved to us that some substance had been separated, the
formation of the gas might easily have escaped our notice, for it is
colourless and transparent like air, and is therefore evolved without
any striking feature. The carbonic acid gas evolved may be weighed 16
and it will be seen that the sum of the weights of the black copper
lo Gas delivery tubes are usually made of glass tubing as prepared at glass works. It
is made of various diameters and thicknesses. If of small diameter and thickness, a glass
tube is easily bent by heating in a gas jet or the flame of a spirit lamp, and may also be
easily divided at a given point by making a deep scratch with a file and then breaking the
tube at this point with a sharp jerk. These properties, together with their impermea-
bility, transparency, hardness, and regularity of bore, makes glass tubes most useful in
experiments with gases. Naturally they might be replaced by straws, india-rubber,
metallic, or other tubes, but these are more difficult to fix on to a vessel, and are not
entirely impervious to gases. A glass gas delivery tube may be hermetically fixed into
a vessel by fitting it into a perforated cork, which should be soft and free from flaws, and
fixing the coi'k into the orifice of the vessel. Sometimes the cork is previously soaked in
paraffin, or it is replaced by an india-rubber cork.
16 Gases, like all other substances, may be weighed, but, owing to their extreme light-
ness and the difficulty of dealing with them in large masses, they can only be weighed by
very sensitive balances ; that is, in such as, with a considerable load, indicate a very small
difference in weight — for example, a centigram or milligram with a load of 1,000 grams.
In order to weigh a gas, a glass globe furnished with a stop- cock (which must not leak in
any part, and therefore must be kept well lubricated) is first of all exhausted of air by an
air-pump la Sprengel pump ia the best). The stop-cock is then closed, and the exhausted
globe weighed. As the pressure of the atmosphere acts on the walls of the globes, they
should be thick. Glass is found to bear the strain of the inequality of the exterior and
interior pressures best. If the gas to be weighed is then let into the globe, its weight
can be determined from the increase in the weight of the globe. It is necessary, how-
ever, that the temperature and pressure of the air about the balance should remain
constant for both weighings, as the weight of the globe in air will (according to the laws
of hydrostatics) vary with its density. The volume of the air displaced, and its weight,
must therefore be determined by observing the temperature, density, and moisture of the
atmosphere during the time of experiment. This will be partly explained later, but may be
studied more in detail by physics. Owing to the complexity of all these operations, the
mass of a gas is usually determined from its volume and density, or the weight of one
volume.
10
PRINCIPLES OF CHEMISTRY
oxide and carbonic acid gas is equal to the weight of the copper car-
bonate17 originally taken, and thus by carefully following out the
various stages of all chemical reactions we arrive at a continuation of
the law of the indestructibility of matter.
3. Red mercury oxide (which is formed as mercury scoria by heat-
ing mercury in air) is decomposed like copper carbonate (only by
heating more slowly and at a somewhat higher temperature), with the
formation of the peculiar gas, oxygen. For this purpose the mercury
oxide is placed in a glass tube or retort,18 to which, by means of a cork,
a gas delivery tube is attached. This tube is bent downwards, as shown
FIG. 1.— Apparatus for the decomposition of red mercury oxide.
in the drawing (Fig. 1). The open end of the gas delivery tube is im-
mersed in a vessel filled with water, called a pneumatic trough.19 When
17 The copper carbonate should be dried before weighing, as otherwise — besides copper
oxide, and carbonic anhydride — water will be obtained in the decomposition. Water
forms a part of the composition of malachite, and has therefore to be taken into considera-
tion. The water produced in the decomposition may be all collected by absorbing it in
sulphuric acid or calcium chloride, as will be described further on. In order to dry a
salt it must be heated at about 100° until its weight remains constant, or be placed under
an air pump over sulphuric acid, as will also be presently described. A* water is met
with almost everywhere, and as it is absorbed by many substances, the possibility of its
presence should never be lost sight of.
18 As the decomposition of red oxide of mercury requires so high a temperature, near
redness, as to soften ordinary glass, it is necessary for the experiment to take a retort
(or test tube) made of infusible (German) glass, which is able to stand high temperatures
without softening. For the same reason, the lamp used must give a strong heat and a
large flame, capable of embracing the whole bottom of the retort, which should be as
small as possible for the convenience of the experiment.
19 The pneumatic trough may naturally be made of any material (china, earthenware)
or metal, &c.), but usually a glass one, as shown in the drawing, is used, as it allows the
progress of experiment being better observed. For this reason, as well as the ease with
which they are kept clean, and from the fact also that glass is not acted on by many sub-
INTRODUCTION
11
the gas begins to be evolved in the retort it is obliged, having no other
outlet, to escape through the gas delivery tube into the water in the
pneumatic trough, and therefore its evolution will be rendered
\ i.sible by the bubbles coming from this tube. In heating the retort
containing the mercury oxide, the air contained in the apparatus is
first partly expelled, owing to its expansion by heat, and then the
peculiar gas called 'oxygen' is evolved, and may be easily collected as it
comes off. For this purpose a vessel (an ordinary cylinder, as in the
drawing) is filled quite full with water and its mouth closed ; it is then
inverted and placed in this position under the water in the trough ;
the mouth is then opened. The cylinder will remain full of water-
that is, the water will remain at a higher level in it than in the sur-
stances which affect other materials (for instance, metals), glass vessels of all kinds —
such a> retorts, test tubes, cylinders, beakers, flasks, globes, &c. — are preferred to any
other for chemical experiments. Glass vessels may be heated without any danger if the
following precautions be observed : 1st, they should be made of thin glass, as otherwise
they are liable to crack from the bad heat-conducting power of glass ; 2nd, they should be
surrounded by a liquid or with sand (Fig. 2), or sand bath as it is called ; or else should
Fiu. 2.- --Apparatus for distillinsr under a diminished pressure liquids which decoiui>ose at their
boiling poim •; under the ordinary pressure. The apparatus in. which the liquid is distilled is con-
oeoted with a large jilobe from which the air is pumped out; the liquid is heated, and the receiver
cr.nl,., I
stand in a current of hot gases without touching the fuel from which they proceed, or in
the flame of a smokeless lamp. A common candle or lamp forms a deposit of soot on a
cold object placed in their flames. The soot interferes with the transmission of heat, and
so a glass vessel when covered with soot often cracks. And for this reason spirit lamps,
which burn with a smokeless flame, or gas burners of a peculiar construction, are used.
In the Bunsen burner the gas is mixed with air, and burns with a non-luminous and
smokeless flame. On the other hand, if an ordinary lamp (petroleum or benzine) does
not smoke it may be used for heating a glass vessel without danger, provided the glass is
placed well above the flame in the current of hot gases. In all cases, the heating should
be begun very carefully by raising the temperature by degrees, and not all at once, or the
glass will break.
12 PKINCIPLES OF CHE3I1STKY
rounding vessel, owing to the atmospheric pressure. The atmosphere
presses on the surface of the water in the trough, and prevents the
water from flowing out of the cylinder. The mouth of the cylinder is
placed over the end of the gas delivery tube,20 and the bubbles
issuing from it will rise into the cylinder and displace the water con-
tained in it. Gases are generally collected in this manner. When a
sufficient quantity of gas has accumulated in the cylinder it can be
clearly shown that it is not air, but another gas which is distinguished
by its capacity for vigorously supporting combustion. In order to show
this, the cylinder is closed, under water, and removed from the bath ;
its mouth is then turned upwards, and a smouldering taper plunged
into it. As is well known, a smouldering taper will be extinguished in
air, but in the gas which is given off from red mercury oxide it burns
clearly and vigorously, showing the capacity this gas has for vigorously
supporting combustion, and thus enabling it to be distinguished from
air. It may be observed in this experiment that, besides the forma-
tion of oxygen, metallic mercury is formed, and, being volatilised at the
high temperature required for the reaction, condenses on the cooler parts
of the retort as a mirror or in globules. Thus two substances, mer-
cury and oxygen, are obtained by heating red mercury oxide. In this
reaction, from one substance two are produced — that is, decomposition
ensues. The means of collecting and investigating gases were already
known before Lavoisier's time, but he first sho.wed the real part they
played in the processes of many chemical changes which before his era
were either wrongly understood (as will be afterwards explained) or were
not explained at all, but only observed in their superficial aspects. This
experiment on red mercury oxide has a special significance in the
history of chemistry contemporary with Lavoisier, because the oxygen
gas which is here evolved is contained in the atmosphere, and plays a
most important part in nature, especially in the respiration of animals,
in combustion in air, and in the formation of rusts or scorise (earths, as
they were then called) from metals — that is, of earthy substances, like the
ores from which metals are extracted. The law of the indestructibility
of matter could not be discovered or confirmed by the balance until the
part played by the atmosphere as regards the participation of its oxygen
in the numerous chemical phenomena, known either from the everyday
experiences of life (combustion, respiration) or from the researches of
20 In order to avoid the necessity of holding the cylinder, its open end is widened (and
also ground so that it may be closely covered with a ground-glass plate when needful), and
placed on a stand below the level of the water in the bath. This stand is called ' the bridge.'
It has several circular openings cut through it, and the gas delivery tube is placed under
one of these, and the cylinder for collecting the gas over it.
INTKoDHTION 13
previous observers (the transformations of the metals into their earths
or oxides), had been explained.
4. In order to illustrate by experiment one more example of
chemical change and the application of the law of the indestructi-
bility of matter, we will take some common table salt and lunar
caustic, which is well known from its use in cauterising wounds. By
taking a clear solution of each and mixing them together, it will at
once be remarked that a solid white substance is formed, which settles
to the bottom of the vessel, and is insoluble in water. This substance
may be separated from the solution by filtering ; it is then found to be
an entirely different substance from either of those taken originally
in the solutions. This is evident from the fact that it does not
dissolve in water. On evaporating the liquid which passed through
the filter, it will be found to contain a new substance unlike either
table salt or lunar caustic, but, like them, soluble in water. Thus
table salt and lunar caustic, two substances soluble in water, being
taken, by their mutual chemical action produced two new substances,
one insoluble in water, and the other remaining in solution. Here,
from two substances two others are obtained, consequently there
occurred a reaction of substitution. The water served only to convert
the acting substances into a liquid and mobile state. If the lunar caustic
and salt be dried 21 and weighed, and if about 58.^ parts by weight — for
instance, grams2'2 — of salt and 170 grams of lunar caustic be taken,
then 143^ grams of insoluble silver chloride and 85 grams of sodium
nitrate will be obtained. The sum of the weights of the acting and
resultant substances are seen to be similar and equal to 228^ grams,
as necessarily follows from the law of the indestructibility of
matter.
21 Drying is necessary in order to remove any water which may be held in the salts
(see Note 17). If the original and resultant substances be dried, then the water
employed for solution, and which is removed in drying, may be taken in indefinite
quantities.
— The exact weights of the acting and resulting substances are determined with the
greatest difficulty, not only from the possible inexactitude of the balance (every weighing
is only correct within the limits of the sensitiveness of the balance) and weights used
in weighing, not only from the difficulty in making corrections for the weight of air dis-
placed by the vessels holding the substances weighed and by the weights themselves,
but also from the hygroscopic nature of many substances (and vessels) causing absorption
of moisture from the atmosphere, and from the difficulty in not losing any of the substance
to be weighed in the many operations (filtering, evaporating, and drying, &c.) which have to
be gone through before arriving at a final result. All these circumstances have to be
taken into consideration in exact ivs.-uivlu's, and their elimination requires very many
special precautions which are impracticable in preliminary experiments ; these arrive-
within only a certain comparatively rough proximity to those weights (expressed by
chemical formulae) which (all with a certain, definite, and inevitable error) correspond
with reality.
14 PRINCIPLES OF CHEMISTRY
Having accepted the truth of the above law, the question in-
voluntarily arises whether there is any limit to the various chemical
transformations, or are they unrestricted in number — that is to say, is
it possible from a given substance to obtain an equivalent quantity of
all other substances ? In other words, does there exist a perpetual and
infinite change of one kind of material into all other kinds, or is the
cycle of these transformations limited ? This is the second essential
problem of Chemistry, a question of quality of matter, and one, it is
-evident, which is more complicated than the question of quantity. It
cannot be resolved by a mere superficial glance at the subject. Indeed,
on seeing how all the varied forms and colours of plants are built up from
air and the elements of the soil, and how metallic iron can be transformed
into dyes, such as inks and Prussian blue, we might be led to think
that there is no end to the qualitative changes to which matter is
susceptible. But, on the other hand, the everyday experiences of life
compel us to acknowledge that food cannot be made out of a stone, or
gold out of copper. Thus a definite answer can only be looked for in
a close and diligent study of the subject, and the problem has been re-
solved in different ways at different times. In ancient times the
opinion most generally held was that everything visible was composed
of four elements — Air, Water, Earth, and Fire. The origin of this
doctrine can be traced far back into the confines of Asia, whence
it was handed down to the Greeks, and most fully expounded by
Empeclocles, who lived before 460 B.C. By accepting so small a
number of elements it was easy to arrive at the conclusion that the
cycle of chemical changes was, if not infinite, at all events most exten-
sive. This doctrine was not arrived at by the results of exact research,
but was only founded on the speculations of philosophers. It appa-
rently owes its origin to the clear division of bodies into gases (like
air), liquids (like water), and solids (like the earth). It seems that
the Arabs were the first who tried to solve the question by means of
experiment, and they introduced, through Spain, the taste for the
study of similar problems into Europe, where from that time there
appear many adepts in chemistry, which was considered as an unholy
art, and called * alchemy.' As the alchemists were ignorant of any
exact or strict law which could guide them in their researches, they re-
solved the question of the transformation of substances in a most varied
manner. Their chief service to chemistry was that they made a
number of experiments, and discovered many new chemical trans-
formations ; but it is well known how they solved the fundamental
problem of chemistry. Their view may be taken as a positive acknow-
ledgment of the infinite transmutability of matter, for they aimed at
INTRODUCTION 15
discovering the Philosopher's Stone, capable of converting everything
into i^'old and diamonds, and of making the old young again. This
solution of the question was afterwards most decidedly refuted, but it
must not, for this reason, be thought that the hopes held by the
alchemists were only the fruit of their imaginations. On the contrary,
the first chemical experiments might well lead them to their conclusion.
They took, for instance, the bright metallic mineral galena, and they
extracted metallic lead from it. Thus they saw that from a metallic
substance which is unfitted for use they could obtain another metallic
substance which is ductile and valuable for many uses in the arts.
Furthermore, they took this lead and obtained silver, a still more
valuable metal, from it. Thus they might easily conclude that it was
possible to ennoble metals by means of a whole series of transmutations
— that is to say, to obtain from them those which are more and more
precious. Having got silver from lead, they only aimed at getting gold
from silver. The mistake they made was that they never weighed or
measured the substances used or produced in their experiments. Had
they done so, they would have learnt that the weight of the lead was
much less than that of the galena from which it was obtained, and the
weight of the silver infinitesimal compared with that of the lead. Had
they looked more closely into the process of the extraction of the silver
from lead (and now silver is chiefly obtained from the lead ores) they
would have seen that the lead does not change into silver, but that it
only contains a certain small quantity of it, and this amount having
once been separated from the lead it cannot by any further operation
give more. The silver which the alchemists extracted from the lead
was in the lead, and was not obtained by a chemical change of the lead
itself. This is now well known from experiment, but the first view of
the nature of the process was very likely to be erroneous.23 The
methods of research adopted by the alchemists could not but give little
23 Besides which, in the majority of cases, the first judgment on most subjects which
do not repeat themselves in everyday experience under various aspects, but always in one
form, or only at intervals and infrequently, is usually untrue. Thus the daily evidence
of the rising of the sun and stars evokes the erroneous idea that the heavens move and
the earth stands f^fcill. This apparent truth is far from being the real truth, and is even
contradictory to it. Similarly, an ordinary mind and everyday experience concludes that
iron is incombustible, whereas it burns not only as filings, but even as wire, as we shall
afterwards see. With the progress of knowledge very many primitive prejudices have
been obliged to give way to true ideas which have been verified by experiment. In ordi-
nary life we often reason at first sight with perfect truth, only because we are taught a
right judgment by our daily experience. It is a necessary consequence of the nature of
our minds to reach the attainment of truth through elementary and often erroneous
reasoning and through experiment, and it would be very wrong to expect a knowledge of
truth from a simple mental effort. Naturally, experiment itself cannot give truth, but it
gives the means of destroying erroneous representations whilst confirming those which
are true in all their consequences.
16 PRINCIPLES OF CHEMISTRY
success, for they groped in the dark, making all kinds of mixtures and
experiments, without setting themselves clear and simple questions
whose answers would aid them to make further pm^ros. Thus they
did not form one exact law, but, nevertheless, they left numerous and
useful experimental data as an inheritance to chemistry ; they studied,
in particular, the transformations proper to metals, and for this reason
chemistry was for long afterwards entirely confined to the study of
metallic substances.
In their researches, the alchemists frequently made use of two
chemical processes which are now termed 'reduction ' and 'oxidation/
The rusting of metals, and in general their conversion from a metallic
into an earthy form, is called ' oxidation,' whilst the extraction of a
metal from an earthy substance is called * reduction.' A large number
of metals — for instance, iron, lead, and tin — are oxidised by heating in
air alone, and may be again reduced by heating with carbon. Such oxi-
dised metals are found in the earth, and form the majority of metallic
ores. The metals, such as tin, iron, and copper, may be extracted from
these ores by heating them together with carbon. All these processes
were well studied by the alchemists. It was afterwards shown that
all earths and minerals are formed of similar metallic rusts or oxides,
or of their combinations. Thus the alchemists knew of two forms of
chemical changes : the oxidation of metals and the reduction of the
oxides so formed into metals. The explanation of the nature of these
two classes of chemical phenomena was the means for the discovery of
the most important chemical laws. The first hypothesis on. their
nature is due to Becker, and more particularly to Stahl, a surgeon to
the King of Prussia. Stahl writes in his * Fundamenta Chymise,'
1723, that all substances consist of an imponderable fiery substance
called ' phlogiston ' (materia aut principium ignis 11011 ipse ignis) and of
another element having particular properties for each substance. The
greater the capacity of a body for oxidation, or the more combustible it
is, the richer it is in phlogiston. Carbon contains it in great abundance.
In oxidation or combustion phlogiston is emitted, and in reduction it
is consumed or enters into combination. Carbon reduces earthy sub-
stances because it is rich in phlogiston, and gives up a portion of its
phlogiston to the substance reduced. Thus Stahl supposed metals to
be compound substances consisting of phlogiston and an earthy sub-
stance or oxide. This hypothesis is distinguished for its very great
simplicity, and for this and other reasons it acquired many supporters.24
a4 It is true that Stahl was acquainted with a fact which directly disproved Ins
hypothesis. It was already known (from the experiments of Geber, and more especially
of Ray, in 1630) that metals increase in weight by oxidation, whilst, according to Stahl's
17
Lavoisier proved by means of tlir balance that every case of rusting
of metals or oxidation, or of combustion, is accompanied by an increase
in -\vi-iirht at the expense of the atmosphere. He formed, therefore, the
natural opinion that the heavier substance is more complex than the
li^hter one.25 The following remarkable experiment wa> madt> by
Lavoisier in 1774, and gave indubitable support to his opinion, which
was iii many respects contradictory to Stahl's doctrine. Lavoisier
hypothesis, they should den-case in weight, because phlogiston is separated l>y oxidation.
Stahl speaks on this point as follows: — 'I know well that metals, in their transformation
into earths, increase in weight. But not only does this fact not disprove my theory, but,
on the contrary, confirms it, for phlogiston is lighter than air, and, in combining with
substances, strives to lift them, and so decreases their weight ; consequently, a substance
which has lost phlogiston must be heavier.' This argument, it will be seen, is founded
on an improper understanding of the properties of gases, regarding them as having no
weight and as not being attracted by the earth, or else on a confused idea of phlogiston
itself, as it was first defined as imponderable. The conception of imponderable phlogiston
tallies well with the habit and methods of the last century, when recourse was often had
to imponderable fluids for explaining a large number of phenomena. Heat, light,
magnetism, and electricity were explained as being peculiar imponderable fluids. In this
sense the doctrine of Stahl corresponds entirely with the spirit of his age. If heat be
now regarded as movement or energy, then phlogiston also should be considered in this
light. In fact, in combustion, of coals, for instance, heat and energy are evolved, and
not combined in the coal, although the oxygen and coal do combine. Consequently, the
doctrine of Stahl contains the essence of a true representation of the evolution of energy,
but naturally this evolution is only a consequence of the combination going on between
the coal and oxygen. As regards the history of chemistry prior to Lavoisier, besides
Stahl's work (to which reference has been made above), Priestley's Experiments and
(>l>nrrr«{t(iitfi an ])i/cri'/it Kin<7s of Air, London, 1790, and also Scheele's Opuscula
Chiinicfi et Phi/sic<i, Lips., 17NS-s(.». '2 vols., must be recommended as the two leading
works of the English and Scandinavian chemists showing the condition of chemical
learning before the propagation of Lavoisier's views. A most interesting memoir on the
history of phlogiston is that of Rodwell, in the Philosophical Magazine, 1868, in which
it is shown that the idea of phlogiston dates very far back, that Basil Valentine (1894-
in. "i, in the Cnrsiis Tn'mii/iJtaJin A/itimonii Paracelsus (1498-1541), in his work, De
Rerun Xnttmt, Glauber (1604-1668), and especially John Joachim Becher (1625-1682), in
his Phi/Nt'cn Siiltfi-nini'd, all referred to phlogiston, but under different names.
25 An Englishman, named Mayow, who lived a whole century before Lavoisier (in 1666),
understood certain phenomena of oxidation in their true aspect, but was not able to
develop his views with clearness, or make his doctrine a universal inheritance, or express
it by instructive experiments ; he, therefore, cannot be considered, like Lavoisier, as
the founder of contemporary chemical learning. Science is a universal heritage, and
therefore it is only just to give the highest honour in science, not to those who first
enunciate a certain truth, but to those who are first able to convince others of its
authenticity and establish it for the general welfare. It should be observed, with refer-
ence to scientific discoveries, that they are rarely made all at once, but, as a rule, the
first teachers do not succeed in convincing others of the truth they have discovered ; with
time, however, the store of materials for its demonstration increases, and other teachers
come forward, possessing every means for making the truth apparent to all. They are
rightly considered as the founders; but it must not be forgotten they are entirely indebted
to the labours and mass of data accumulated by many others. Such was Lavoisier, and
such an- all the great founders of science. They are the enunciators of all past and
• lit learning, and their names will always be revered by posterity.
VOL. I. C
18 PRINCIPLES OF CHKMISTKY
poured four ounces of pure mercury into a glass retort (fig. 3), whose
neck was bent as shown in the drawing and dipped into the vessel R s,
also full of mercury. The projecting end of the neck was covered
with a glass bell jar P. The weight of all the mercury taken, and the
volume of air remaining in the apparatus, namely, that in the upper
portion of the retort, and under the bell-jar, were determined before
beginning the experiment. In this experiment it was most important
to know the volume of air in order to learn what part it played in the
oxidation of the mercury, because, according to Stahl, phlogiston is
emitted into the air, whilst, according to Lavoisier, the mercury in
FIG. 3. — Lavoisier's apparatus for determining the composition of air and the
reason of metals increasing in weight when they are calcined in air.
oxidising absorbs a portion of the air ; and consequently it wras abso-
lutely necessary to determine whether the amount of air increased or
decreased in the oxidation of the metal. It was, therefore, most import-
ant to measure the volume of the air in the apparatus both before and
after the experiment. For this purpose it was necessary to know the
total capacity of the retort, the volume of the mercury poured into it,
the volume of the bell-jar above the level of the mercury, and also
the temperature and pressure of the air at the time of its measure-
ment. The volume of air held in the apparatus and isolated from the
surrounding atmosphere could be determined from these data. Having
arranged his apparatus in this manner, Lavoisier heated the retort
holding the mercury for a period of twelve days at a temperature near
the boiling point of mercury. The mercury became covered with a
quantity of small red scales ; that is, it was oxidised or converted into
an earth. This substance is the same mercury oxide which has already
been mentioned (example 3). After the lapse of twelve days the
apparatus was cooled, and it was then seen that the volume of the air
in the apparatus had diminished during the time of the experiment.
This result was in exact contradiction to Stahl's hypothesis. Out
of 50 cubic inches of air originally taken, there only remained 42.
INTKnIMVTION 19
Lavoisier's experiment led to other no less important results. The
weight of the air taken decreased by as much as the weight of the
mercury increased in oxidising ; that is, the portion of the air was not
destroyed, but only combined with mercury. This portion of the air
may be again separated from the mercury oxide, and has, as we saw
(example 3), properties different from those of air. That portion of
the air which remained in the apparatus and did not combine with the
mercury does not oxidise metals, and cannot support either combus-
tion or respiration, so that a lighted taper is immediately extinguished
if it be dipped into the gas which remains in the bell-jar. * It is ex-
tinguished in the remaining gas as if it had been plunged into water/
writes Lavoisier in his memoirs. This gas is called ' nitrogen.5 Thus
air is not a simple substance, but consists of two gases, oxygen and
nitrogen, and therefore the opinion that air is an elementary substance
is erroneous. The oxygen of the air is absorbed in combustion and the
oxidation of metals, and the earths produced by the oxidation of
metals are substances composed of oxygen and a metal. By mixing
the oxygen with the nitrogen the same air as was originally taken is
re-formed. The existence of compound substances was incontestably
proved by these experiments. It has also been shown by direct experi-
ment that on reducing an oxide with carbon, the oxygen contained
in the oxide is transferred to the carbon, and gives the same gas as is
obtained by the combustion of carbon in air. Therefore this gas is
a compound of carbon and oxygen, just as the earthy oxides are com-
posed of metals and oxygen.
The many examples of the formation and decomposition of sub-
stances which are met with convince us that the majority of substances
with which we have to deal are compounds made up of several other
substances. By heating chalk (or else copper carbonate, as in the
second example) we obtain lime and the same carbonic acid gas which is
produced by the combustion of carbon. On bringing lime into contact
with this gas and water, at the ordinary temperature, we again obtain the
compound carbonate of lime, or chalk. Therefore chalk is a compound.
So also are those substances from which it may be built up. Car-
bonic anhydride is formed by the combination of carbon and oxygen ;
and lime is produced by the oxidation of a certain metal called ' cal-
cium.' By breaking up substances in this manner into their component
parts, we arrive at last at such as are indivisible into two or more sub-
stances by any means whatever, and which cannot be formed from other
substances. All we can do is to make such substances combine together
or act on other substances. Substances which cannot be formed from or
decomposed into others are termed simple substances (elements). Thus
c 2
20 PRINCIPLES OF CHEMISTRY
all homogeneous substances maybe classified into simple and compound
substances. This view was introduced and established as a scientific
fact during the lifetime of Lavoisier. The number of these elements
is very small in comparison with the number of compound substances
which are formed by them. At the present time, only seventy elements
are known with certainty to exist. Some of them are very rarely met
with in nature, or are found in very small quantities, whilst others
are yet doubtful. The number of elements with whose compounds we
commonly deal in everyday life is very small. Elements cannot be
transmuted into one another — at least up to now not a single ,-ase of
such a transformation has been met with ; it may therefore be said
that, as yet, it is impossible to transmute one metal into another. And
as yet, notwithstanding the number of assays which have been made in
this direction, no fact has been discovered which could in any way
support the idea of the complexity of those indubitably-known ele-
ments 26 — such as oxygen, iron, sulphur, &c. Therefore, from its con-
ception, an element is not susceptible to reactions of decomposition.-7
-fi Many ancient philosophei's admitted the existence of one elementary form of
matter. This idea still appears in our times, in the constant efforts \\hicli are made to
reduce the number of the elements; to prove, for instance, that bromine contains chlorine
or that chlorine contains oxygen. Many methods, founded both on experiment and
theory, have been tried to prove the compound nature of the elements. All labour" in
this direction has as yet been in vain, and the assurance that elementary matter is not
so homogeneous (single) as the mind would desire in its first transport of rapid generali-
sation is strengthened from year to year. At all events, there are as yet no experimental
or theoretical evidences of the compound nature of our elements. With the methods
and evidence now at our disposal it is impossible to even imagine the possibility of a
method by which the different elements could be formed from one elementary material.
Cases of isomerism and of polymerism of compound substances certainly show the pos-
sibility of the formation, from one and the same elements, of substances with different
properties, but every change of this kind is completely levelled and nullified by a certain
rise in temperature by which every isomeride and polymeride is converted into one
variety and changes its original properties All our knowledge Allows that iron and
other elements remain, even at such a high temperature as there exists in the sun. as
different substances, and are not converted into one common material. Admitting, even
mentally, the possibility of one elementary form of matter, a method must lie imagined
by which it could give rise to the various elements, as also the )nt><ln* o/it'r<ni(li of their
formation from one material. If it be said that this diversitude only takes place at low
temperatures, as is observed with isomerides, then there would be reason to expect, if not
the transition of the various elements into one particular and more stable form, at least
the mutual transformation of some into others. But nothing of the kind has yet been
observed, and the alchemist's hope to manufacture (as Berthollet puts it) elements has no
foundation of fact or theory.
27 The weakest point in the idea of elements is the negative character of the determi-
native signs given them by Lavoisier, and from that time ruling in chemistry. They do
no^decompose, they do not change into one another. But it must be remarked that
elements form the limiting horizon of our knowledge of matter, and it is always difficult
to determine a positive side on the borderland of what is known. But all the same, if
not for all, at all events for the majority, of those having the properties of metals, there
is a series of positive common signs (they possess a particular appearance and lustre,
21
The quantity, therefore, <»f each clement remains constant in all
chemical changes ; which fact may be deduced as a consequence of the
la\\ of the indestructibility of matter, and uf the conception of elements
themselves. Thus the equation expressing the law of the indestructi-
bility of matter acquires a new and still more important signilication.
If we know the quantities of the elements which occur in the acting,
it may be compound, substances, and if from these substances there
proceed, by means of chemical changes, a series of new compound sub-
stances, then the latter will together contain the same quantity of each
of the elements as there originally existed in the reacting substances.
The essence of chemical change is embraced in the study of how,
and with what substances, each element is combined before and after
change.
In order to be able to express various chemical changes by equations,
it has been agreed to represent each element by the first or some two
letters of its (Latin) name. Thus, for example, oxygen is represented by
the letter O ; nitrogen by N ; mercury (hydrargyrum) by Hg ; iron
(ferrum) by Fe ; and so on for all the elements, as is seen in the tables
on page 24. A compound substance fe represented by placing the
symbols representing the elements of which it is made up side by side.
For example, red mercury oxide is represented by HgO, which shows
that it is composed of oxygen and mercury. Besides this, the symbol
of every element corresponds with a certain relative quantity of it by
weight, called its ' combining ' weight, or the weight of an atom; so that
the chemical formula of a compound substance not only designates the
nature of the elements of which it is composed, but also their quantita-
tive proportion. Every chemical process may be expressed by an equa-
tion composed of the formulae corresponding with those substances
which take part in it and are produced by it. The amount by weight
of the elements in every chemical equation must be equal on both sides
of the equation, because no element is either formed or destroyed in a
chemical change.
On pages 24, 25, and 26 a list of the elements, with their symbols
and combining or atomic weights, is given, and we shall see afterwards
on what basis the atomic weights of elements are determined. At
present we will only point out that a compound containing the elements
A and B is designated by the formula AMB"1, where m and n are the
coefficients or multiples in which the combining weights of the
they conduct an electric current without decomposing) which allow them to be distin-
guished at a glance from other kinds of matter. Besides, there is no doubt (from the
results of spectrum analysis) that the elements are distributed as far as the most
distant stars, and -that they support the highest attainable temperatures without
decomposing.
22 PRINCIPLES OF CHEMISTRY
elements enter into the composition of the substance. If we repre-
sent the combining weight of the substance A by a and that of the
substance B by 6, then the composition of the substance A"B'" will be
expressed thus : it contains na parts by weight of the substance A and
nib parts by weight of the substance B, and consequently in 100 parts
of our compound there is contained n percentage parts by weight
of the substance A and ^— of the substance B. It is evident that
na-}- mo
as a formula shows the relative amounts of all the elements contained
in a compound, the actual weights of the elements contained in a given
weight of a compound may be calculated from its formula. For example,
the formula NaCl of table salt shows (as Na=23 and Cl = 35'5), that 58'5
Ibs. of salt contain 23 Ibs. of sodium and 35'5 Ibs. of chlorine, and that 100
parts of it contain 39 -3 per cent, of sodium and 60*7 per cent, of chlorine.
What has been said above clearly limits the province of chemical
changes, because from substances of a given kind there can be obtained
only such as contain the same elements. But, notwithstanding this
primary limitation, the number of possible combinations is infinitely
great. Only a comparatively small number of compounds have yet
been described or subjected to research, and any one working in this
direction may easily discover new compounds which had not before
been obtained. It often happens, however, that such newly -discovered
compounds were foreseen by chemistry, whose object is the apprehension
of that uniformity which rules over the multitude of compound sub-
stances, and whose aim is the comprehension of those laws which govern
their formation and properties. When once the conception of ele-
ments had been established, the most intimate object of chemistry
was the determination of the properties of compound substances on the
basis of the determination of the quantity and kind of elements of
which they are composed ; the investigation of the elements themselves;
the determination of what compound substances can be formed from
each element and the properties which these compounds show ; and the
apprehension of the nature of the connection between the elements in
different compounds. An element thus serves as the starting point,
and is taken as the primary conception under which all other bodies
are embraced.
When we state that a certain element enters into the composition
of a given compound (when we say, for instance, that mercury oxide
contains oxygen) we do not mean that it contains oxygen as a gaseous
substance, but only desire to express those transformations which
mercury oxide is capable of making ; that is, we wish to say that it is
INTRODUCTION 23
possible to obtain oxyi^-ii from mercury oxide, and that it can -i\«-
up oxygen to various other substances ; in a word, we desire only to
express those transformations of which mercury oxide is capable. Or,
more concisely, it may be said that the mmjHtxifion of a compound is
the expression of those transformations of which it is capable. It is
useful in this sense to make a clear distinction between the conception
of an element as a x'//'//v/v homogeneous substance, and as a material,
but invisible part of a compound. Mercury oxide does not contain
two simple bodies, a gas and a metal, but two elements, mercury and
oxygen, which, when free, are a gas and a metal. Xeither mercury as a
metal nor oxygen as a gas is contained in mercury oxide ; it only contains
the substance of these elements, just as steam only contains the sub-
stance of ice, but not ice itself, or as corn contains the substance of the
seed but not the seed itself. The existence of an element may be recog
nised without knowing it in the uncombined state, but only from an in-
vestigation of its combinations, and from the knowledge that it gives,
under all possible conditions, substances which are unlike other known
combinations of substances. Fluorine is an example of this kind. It
was for a long time unknown in a free state, and was, nevertheless, recog-
nised as an element because its combinations with other elements were
known, and their difference from all other similar compound substances
was determined. In order to grasp the difference between the con-
ception of the visible form of an element as we know it in the free
state, and of the intrinsic element (or * radicle,' as Lavoisier called it)
contained in the visible form, it should be remarked that compound
substances also combine together forming yet more complex compounds,
and that they evolve heat in the process of combination. The original
compound may often be extracted from these new compounds by exactly
the same methods as elements are extracted from their corresponding
combinations. Besides, many elements exist under various visible forms
whilst the intrinsic element contained in these various forms is some-
thing which is not subject to change. Thus carbon appears as charcoal,
graphite, and diamond, but yet the element carbon alone contained in
each is one and the same. Carbonic anhydride contains carbon, and
not charcoal, or graphite, or the diamond.
Elements alone, although not all of them, have the peculiar lustre,
opacity, malleability, and the great heat and electrical conductivity
which are proper to metals and their mutual combinations. But
elements are far from all being metals. Those which do not possess
the physical properties of metals are called in>n-ntcft(fi< (or metalloids).
It is, however, impossible to draw a strict line of demarcation between
metals and non-metals, there being many intermediary substances.
•24
PRINCIPLES OF CHEMISTRY
Thus graphite, from which pencils are manufactured, is an element
with the lustre and other properties of a metal ; but charcoal and the
diamond, which are composed of the same substance as graphite, do
not show any metallic properties. Both classes of elements are clearly
distinguished in definite examples, but in particular cases the distinc-
tion is not clear and cannot serve as a basis for the exact division of
the elements into two groups.
At all events, the conception of elements forms the basis of chemical
knowledge, and if we give a list of them at the very beginning of our
work, it is that we wish to symbolise the condition of the contemporary
information on the subject. Altogether about seventy elements are
now authentically known, but many of them are so rarely met with in
nature, and have been obtained in such small quantities, that we possess
but a very insufficient knowledge of them. The substances most widely
distributed in nature contain a very small number of elements. These
elements have been more completely studied than the others because a
greater number of investigators have been able to carry on experiments
and observations on them. The elements most widely distributed in
nature are : —
Hydrogen, H =1. In water, and animal and vegetable or-
ganisms.
Carbon, C =12. In organisms, coal, limestones.
Nitrogen, N =14. In air and in organisms.
Oxygen, O =16. In air, water, earth. It forms the greater
part of the mass of the earth. "
In common salt and in many minerals.
In sea-water and in many minerals.
In minerals and clay.
In sand, minerals, and clay.
In bones, ashes of plants, and soil.
In pyrites, gypsum, and in sea- water.
In common salt, and in the salts of MM
water.
K =39. In minerals, ashes of plants, and in nitre.
Ca = 40. In limestones, gypsum, and in organisms.
Fe =56. In the earth, iron ores, and in organisms.
Beside these, the following elements, although not very largely dis-
tributed in nature, are all more or less well known from their applicati< >ns
to the requirements of everyday life or the arts, either in a free state
or in their compounds : —
Lithium, Li =7. In medicine (Li.2C03), and in photography (LiBr).
Boron, B=l 1. As Borax, B4Na2O7, and as boric anhydride, B2O3.
Sodium, Na=23.
Magnesium, Mg = 24.
Aluminium, Al =27.
Silicon, Si =28.
Phosphorus,? =31.
Sulphur,
Chlorine,
Potassium,
Calcium,
Iron,
S
Cl
= 32.
=35-5.
= 39.
[INTRODUCTION
25
Fluorine, F =19.
Chromium, Cr =-r>2.
Maiiu-anc.se, M M=")").
Co=i>9.
Cobalt,
Nickel,
Copper,
Zinc,
Arsenic,
Bromine,
Cu=
Zn=
AS:
Br =
Strontium, Si-
Silver, AO
Cadmium, Cd
Tin, Sn
Antimony, Sb
Iodine, I
=63.
:<;:>.
:7">.
= 80.
= 87.
= 112.
= 118.
= 122.
= 127.
Barium, Ba = 137.
Platinum, Pt =196.
Gold, Au=197.
Mercury, Hg=200.
Lead, ' Pb=207.
Bismuth, Bi =208.
Uranium, U =240.
As fluor spar. Cal%, and as hydrofluoric
,u id, HF.
As chromic anhydride, CrO3, and potas-
sium dichromate, K2Cr2O7.
As manganese peroxide, Mn02, and po-
tassium permanganate, MnKO4.
In smalt and blue glass.
For electro-plating other metals.
The well-known red metal.
Used for the plates of batteries, roofing, &c.
White arsenic, As203.
A browTii volatile liquid ; sodium bromide,
NaBr.
In coloured tires (SrN,O6).
The well-known white metal.
In alloys. Yellow paint (CdS).
The well-known metal.
In alloys such as type metal.
In medicine and photography ; free, and as
KI.
" Permanent white," and as an adulterant
in white lead, and in heavy spar, BaS04.
^Well-known metals.
)
In medicine and fusible alloys.
In green fluorescent glass.
The compounds of the following metals and semi-metals have fewer
applications, but are well known, and are somewhat frequently met
with in nature, although in small quantities : —
Palladium, Pd=106.
Cerium, Ce=140.
Tungsten, W =184.
Osmium, Os=193.
Iridium, Ir=195.
Thallium, Tl=204.
Beryllium,
Be =9.
Titanium,
Ti =48.
Vanadium,
V =51.
Selenium,
Se =78.
Zirconium,
Zr =90.
Molybdenum, Mo =.96.
The following rare metals are still more seldom met with in nature
and are not yet applied to the arts, but have been studied somewhat
fully :—
26 PRINCIPLES OF CHEMISTRY
Scandium, Sc =44. Indium, In =11 3,
Gallium, Ga=68. Tellurium, Te = 12.r>.
Germanium, Ge= 72. Caesium, Cs=132.
Rubidium, Rb=S5. Lanthanum, La]=138.
Yttrium, Y =89. Didymium, Di =143.
Niobium, Nb=94. Ytterbium, Yb=173.
Ruthenium, Ru=104. Tantalum, Ta =182.
Rhodium, Rh= 1 04. Thorium, Th = 234.
Besides these 66 elements there have been discovered : — Erbium,
Terbium, Samarium, Thallium, Holmium, Mosandrium, Phillipium,
Vesbium, Actinium, and several others. But their properties and com-
binations, owing to their extreme rarity, are very little known, and even
their existence as independent substances 28 is doubtful.
It has been incontestably proved from observations on the spectra
of the heavenly bodies that many of the most common elements (such
as H, Na, Mg, Fe) occur on the far distant stars. This fact confirms
the belief that those forms of matter which appear on the earth as
elements are widely distributed over the entire universe. But why,
in nature, the mass of some elements should be greater than that of
others we do not yet know.
The capacity of each element to combine with one or another
element, and to form compounds with them which are in a greater or
less degree prone to give new and yet more complex substances, forms
the fundamental character of each element. Thus sulphur easily com-
bines with the metals, oxygen, chlorine, or carbon, forming stable sub-
stances, whilst gold and silver enter into combinations with difficulty,
and form unstable compounds, which are easily decomposed by heat.
Compounds, and also elements, may be divided into two classes — those
which easily enter into many different chemical changes, and those which
enter into but few combinations, which are characterised by their small
capacity for the direct formation of new, more complex substances.
The cause or force which induces substances to enter into chemical
change must be considered, as also the cause which holds different
substances in combination — that is, which endues the substances
formed with their particular degree of stability. This cause or force
is called affinity (affinitns, affinite, verwandtsckaft), or chemical affinity.29
28 It may be that some of them are compounds of other already-known elements.
Pure and incontestably independent compounds of these substances are unknown, and
some of them have not even been separated but are only supposed to exist from the
results of spectroscopic researches. There can be no mention of such contestalilc and
doubtful elements in a short general handbook of chemistry.
29 This word, first introduced, if I mistake not, into chemistry by Glauber, is based on
the idea of the ancient philosophers that combination can only take place when the sub-
iNTi;oi»rcTiON 27
As this t'< >ivc must be regarded as exclusively an Attractive force,
like gravity, many writers (for instance, Berginanii at the end of the
last, and Berthollet at the beginning of this, century) supposed affinity
to be essentially similar to the universal force of gravity, from which
it only differs in that the latter acts at observable distances whilst
affinity only evinces itself at the smallest possible distances. But
chemical affinity cannot be entirely identified with the universal
at traction of gravity, which acts at observable distances and which
is dependent only on mass and distance, and not on the quality of the
material on which it acts, whilst it is by the quality of matter that
affinity is most forcibly influenced. Neither can it be entirely identi-
fied with cohesion, which gives to homogeneous solid substances their
crystalline form, elasticity, hardness, ductility, and other properties,
and to liquids their surface, drop formation, capillarity, and other
properties, because affinity acts between the component parts of a
substance and cohesion on a substance in its homogeneity, although
both act at imperceptible distances (by contact) and have much in
common. Chemical force, which makes one substance penetrate into
another, cannot be entirely identified with even those attracting
forces which make different substances adhere to each other, or hold
together (as when two plane-polished surfaces of solid substances are
brought into close contact), or which cause liquids to soak into solids,
or adhere to their surfaces, or gases and vapours to condense on the sur-
faces of solids. These forces must not be confounded with chemical
forces, which cause one substance to .penetrate into the substance of
another and to form a new substance, which is not the case with
cohesion. But it is evident that the forces which determine cohesion
form a connecting-link between mechanical and chemical forces, be-
cause they only act by intimate contact and between different kinds of
matter. For a long time, and especially during the first half of this
century, chemical attraction and chemical forces were identified with
electrical forces. There is certainly an intimate relation between them,
for electricity is evolved in chemical reactions, and it, in its turn, has
a powerful influence on chemical processes — for instance, compounds
are decomposed by the action of an electrical current. But the exactly
similar relation which exists between chemical phenomena and the
phenomena of heat (heat being developed by chemical phenomena, and
heat being able to decompose compounds) only proves the unity of the
forces of nature, the capability of one force to produce and to be trans-
stances combining have something in common — a medium. As is generally the case,
another idea evolved itself in antiquity, and has lived until now, side by side with the
first, to which it is exactly contradictory ; this considers union as dependent on con-
trast, on polar difference, on an effort to fill up a want.
28 PRINCIPLES OF CHEMISTRY
formed into others. Therefore the identification of clu inical force with
electricity will not bear experimental proof.30 As of all the (mole-
cular) phenomena of nature which act on substances at immeasurably
small distances, the phenomena of heat are at present the best (com-
paratively) known, having been reduced to the simplest fundamental
principles of mechanics (of energy, equilibrium, and movement), which,
since Newton, have been subjected to strict mathematical analysis,
it is quite natural that an effort, which has been particularly
pronounced during recent years, should have been made to bring
chemical phenomena into strict correlation with, and under the theory
founded on, the already investigated phenomena of heat, without, how-
ever, aiming at any identification of chemical with heat phenomena.
The true nature of chemical force is still a secret to us, just as is the
nature of the universal force of gravity, and yet without knowing what
gravity really is, by applying mechanical conceptions, astronomical
phenomena have been subjected not only to exact generalisation but to
the detailed prediction of a number of particular facts ; and so, also,
although the true nature of chemical affinity may be unknown, there
is reason to hope for considerable progress in chemical science by
applying the laws of mechanics to chemical phenomena by means of
the mechanical theory of heat. But as yet this portion of chemistry
has been but little worked at, and therefore, while forming a current
problem of the science, it is treated more fully in that particular
50 Especially conclusive are those cases of so-called metalepsis (Dumas, Laurent).
Chlorine, in combining with hydrogen, forms a very stable substance, called ' hydrochloric
acid,' which is split up by the action of an electrical current into chlorine and hydrogen,
the chlorine appearing at the positive and the hydrogen at the negative pole. From this
electro-chemists considered hydrogen to be an electro-positive and chlorine an electro-
negative element, and that they are held together in virtue of their opposite electric
charges. It appears, however, from metalepsis, that chlorine can replace hydrogen (and
reversely hydrogen replaces chlorine) in its compounds without in any way changing the
grouping of the other elements, or altering their chief chemical properties. Thus the
capacity of acetic acid to form salts is not altered by replacing its hydrogen by chlorine.
Here an electro-positive element is replaced by an electro-negative element, which is
quite contrary to the electrical theory of the origin of chemical attraction, which has thus
been entirely overthrown by the facts of metalepsis. We must remark, whilst consider-
ing this subject, that the explanation suggesting electricity as the origin of chemical
phenomena is unsound in that it strives to explain one class of phenomena whose nature
is almost unknown by another class which is no better known. It is most instructive to
remark that together with the electrical theory of chemical attraction there arose and
survives a view which explains the galvanic current as being a transference of chemical
action through the circuit — i.e., regards the origin of electricity as being a chemical one. It
is evident that the connection is very intimate, but both kinds of phenomena are indepen-
dent and represent different forms of molecular (atomic) movement, whose real nature is
not yet understood. Nevertheless, the connection between the phenomena of both cate-
gories is not only in itself very instructive, but it extends the applicability of the general
idea of the unity of the forces of nature, conviction of the truth of which has held so
important a place in the science of the last ten years.
province which is termed either 'theoretical' or 'physical' chemistry, or,
better still, flo'ni'n-nl m^-hnnics. As this province of chemistry re-
quires a knowledge not only of the various homogeneous substances
which have yet been obtained and of the chemical transformations which
they undergo, but also of the phenomena (of heat and other kinds) by
which these transformations are accompanied, it is only possible to
• •nter on the study of chemical mechanics after an acquaintance with
the fundamental chemical conceptions and substances which form the
subject of this book.31
r>1 I consider that in an elementary textbook of chemistry, like the present, it is only
possible and advisable to mention, in reference to chemical mechanics, a few general
ideas and some particular examples referring more especially to gases, whose mechanical
theory must be regarded as the most complete. The molecular mechanics of liquids and
solids is as yet in embryo, and contains much that is disputable; for this reason,
chemical mechanics has made less progress in relation to these substances. It may not
be superfluous to here remark, with respect to the conception of chemical affinity, that up
to the present time gravity, electricity, and heat have been respectively applied to its
elucidation. Efforts have also been made to introduce the luminiferous ether into
theoretical chemistry, and should that connection between the phenomena of light and
electricity which was established by Maxwell be worked out more in detail, doubtless
these efforts to elucidate all or a great deal by the aid of luminiferous ether will yet again
appear in theoretical chemistry. An independent chemical mechanics of the material
particles of matter, and of their internal (atomic) changes, would, in my opinion, arise a-
the result of these efforts. Just as the progress made in chemistry in the time of
Lavoisier was reflected over all natural science, so there is reason to think that an in-
dependent chemical mechanics would shed a new light on all molecular mechanics, which
must be considered as the fundamental problem of the exact sciences in our times. Two
hundred years ago Newton laid the foundation of a truly scientific theoretical mechanics
of extemal visible movement, and erected the edifice of celestial mechanics on this
foundation. One hundred years ago Lavoisier arrived at the first fundamental law of the
internal mechanics of invisible particles of matter. This subject is far from having been
developed into a harmonious whole, because it is much more difficult, and, although many
details have been completely investigated, it does not possess any starting points.
Newton was possible only after Copernicus and Kepler, who had discovered the exte-
rior empirical simplicity of celestial phenomena. Lavoisier and Dalton may, in respect
to the chemical mechanics of the molecular world, be compared to Copernicus and
Kepler. But a Newton has not yet appeared in the molecular world ; when he does, I
think that he will find the fundamental laws of the mechanics of the invisible movements
of matter more easily and more quickly in the chemical structure of matter than in
physical phenomena (of electricity, heat, and light), for these latter are accomplished by
already-disposed particles of matter, whilst it is now clear that the problem of chemical
mechanics mainly lies in the apprehension of those movements which are invisibly ac-
complished by the smallest atoms of matter. The general laws of mechanics, established
by Newton, will probably serve as starting points for molecular mechanics, but the
independence of its range becomes more evident when chemical molecules are com-
pared with the celestial systems, such as the solar system. Chemical atoms may be
regarded as separate members of such systems (as, for instance, the sun, planets, comets,
and other heavenly bodies), whilst the ether of light may be likened to the cosmic dust
which without doubt is distributed throughout space. The present condition of molecular
mechanics is, to a certain extent, copied from celestial mechanics, but there is nothing to
prove the entire similarity of both worlds, although it appears to the mind that, starting
from the primary elements of the unity of creation, such a representation is the most
likelv.
30 PRINCIPLES OF CHEMISTUY
As the chemical changes to which substances are liable proceed
from internal forces proper to these substances, as chemical phenomena
certainly consist of movements of material parts (from the laws of the
indestructibility of matter and of elements), and as the investigation
of mechanical and physical phenomena proves the law of the indestruc-
tibility of forces, or the conservation of energy — that is, the possibility
of the transformation of one kind of movement into another (of visible
or mechanical into invisible or physical) — we are inevitably obliged to
acknowledge the presence in substances (and especially in. the elements
of which all others are composed) of a store of chemical energy or in-
visible movement inducing them to enter into combinations. If heat be
evolved in a reaction, it means that a portion of chemical energy is
transformed into heat ; 32 if heat be absorbed in a reaction,33 that it is
32 The theory of heat gave the idea of a store of internal movement or energy, and
therefore with it, it became necessary to acknowledge chemical energy, but there is no
foundation whatever for identifying heat energy with chemical energy. It may be sup-
posed, but not positively affirmed, that heat movement is proper to molecules and
chemical movements to atoms, but that as molecules are made up of atoms, the movement
of the one passes to the other, and that for this reason heat strongly influences reaction
and appears or disappears (is absorbed) in reactions. These relations, which are,
apparent and hardly subject to doubt on general lines, still present much that is doubtful
in detail, because all forms of molecular and atomic movement are able to pass into
each other. On broad general lines it must be acknowledged that as mechanical energy
can entirely pass into heat energy (but the reverse transition is accomplished only
partially, according to the second law of heat), so also heat energy may pass into
chemical energy, but it is doubtful, and even unlikely, that chemical energy passes
altogether into heat energy. Therefore, the heat evolved in chemical reactions cannot
serve as the total measure of chemical energy, more especially as there are a number of
reactions of combination in which heat is absorbed ; for instance, the combination of
charcoal with sulphur is accompanied by an absorption of heat — probably because the
molecules of charcoal are complex, and those of carbon bisulphide less so, and the break-
ing up of the complex molecules of charcoal requires a large absorption of heat (whose
measure we do not know) — and whilst the combination of charcoal with sulphur is accom-
panied by an evolution of heat, yet we only observe the difference of these two heat
effects.
33 The reactions which take place (at the ordinary or at a high temperature) directly
between substances may be clearly divided into exothermal, which are accompanied by
an evolution of heat, and endothermal, which are accompanied by an absorption of heat.
It is evident that the latter require a source of heat. They are determined either by the
directly surrounding medium (as in the formation of carbon bisulphide from charcoal and
sulphur, or in decompositions which take place at high temperatures), or else by a
simultaneously proceeding secondary reaction. So, for instance, hydrogen sulphide is
decomposed by iodine in the presence of water at the expense of the heat which is
evolved by the solution in water of the hydrogen iodide produced. This is the reason why
this reaction, as exothermal, only takes place in the presence of water ; otherwise it would
be accompanied by a cooling effect. As in the combination of dissimilar substances, the
bonds existing between the molecules and atoms of the homogeneous substances have to
be broken asunder, whilst in reactions of rearrangement the formation of any one sub-
stance proceeds parallel with the formation of another, and, as in reactions, a series of
physical and mechanical changes take place, it is impossible to separate the heat directly
depending on a given reaction from the total sum of the observed heat effect. For this
nN 31
partly transformed (rendered latent) into chemical energy. The store
of force or energy going to the formation of new compounds may, after
several combinations, accomplished with an absorption of heat, at last
diminish to such a degree that indifferent compounds will be obtained,
although these sometimes, by combining with energetic elements or
compounds, give more complex compounds, which may be capable of
entering into chemical combination. Among elements gold, platinum,
and nitrogen have but little energy, whilst potassium, oxygen, and
chlorine have a very marked degree of energy. When dissimilar sub-
stances enter into combination they often form substances of diminished
energy. Thus sulphur and potassium when heated easily burn in air,
but when combined together their compound is neither inflammable nor
burns in air like its component parts. Part of the energy of the
potassium and of the sulphur was evolved in their combination in the
form of heat. Just as in the passage of substances from one physical
state into another a portion of their store of heat is absorbed or
evolved, so in combinations or decompositions and in every chemical
process, there occurs a change in the store of chemical energy, and at
the same time an evolution or absorption of heat.34
For the comprehension of chemical phenomena in a mechanical
sense — i.e., in the study of the modus operandi of chemical phenomena-
it is at the present time most important to consider : (1) the facts
gathered from stoichiometry, or that part of chemistry which treats of
the quantitative relation, by weight or volume, of the. reacting sub-
stances ; (2) the distinction between the different forms and classes of
chemical reactions ; (3) the study of the changes in properties produced
by alteration in composition ; (4) the study of the phenomena which
accompany chemical transformation ; (5) a generalisation of the con-
ditions under which reactions occur. As regards stoichiometry, this
branch of chemistry has been worked out most thoroughly, and embraces
laws (of Dalton, A vogadro- Gerhard t, and others) which bear so deeply
on all parts of chemistry that its entire contemporary standing may be
reason, thermo-chemical data are very complex, and cannot by themselves give the key
to many chemical problems, as it was at first supposed they might. They ought to form
a part of chemical mechanics, but alone they do not constitute it.
3* As chemical reactions are effected by heating, so the heat absorbed by substances
before decomposition or change of state, and called ' specific heat,' goes in many cases to the
preparation, if it may be so expressed, of reaction, even when the limit of the temperature
of reaction is not attained. The molecules of a substance A, which is able to react on a
substance B below a temperature t by being heated from a somewhat lower temperature to
/, undergoes that change which had to be arrived at for the formation of A B. This
idea is often extended ; for instance, it is supposed that a given sul>-tance in its passage
from a liquid to a gaseous state gives chemically or materially new, lighter, and simpler
molecules (is depolymerised, according to De Haen).
32
characterised as the epoch of their circumstantial application to par-
ticular cases. The expression of the quantitative (volumetric or gravi-
metric) composition of substances now forms the most important pro-
blem of chemical research, and therefore the entire further exposition
of the subject is subordinate to stoichiometrical laws. All other
branches of chemistry are clearly subordinate to this most important
portion of chemical knowledge. Even the very signification of re-
actions of combination, decomposition, and rearrangement, acquired, as
we shall see, a particular and new character under the influence of the
progress of exact ideas concerning the quantitative relations of sub-
stances entering into chemical changes. Furthermore, in this sense
there arose a new — and, up to then, unknown --division of compound
substances into definite and indefinite compounds. Even at the beginning
of this century, Berthollet had not made this distinction. But Prout
showed that a number of compounds contain the substances of which
they are composed and into which they break up, in exact definite pro-
portions by weight, which are unalterable under any conditions. Thus,
for example, red mercury oxide contains sixteen parts by weight of
oxygen for every 200 parts by weight of mercury, which is expressed
by the formula HgO. But in an alloy of copper and silver one or the
other metal may be added at will, and in an aqueous solution of sugar,
the relative proportion of the sugar and water may be altered and
nevertheless a homogeneous whole with the sum of the independent
properties will be obtained — i.e., in these cases there was indefinite
chemical combination. Although in nature and chemical practice the
formation of indefinite compounds (such as alloys and solutions) plays
as essential a part as the formation of definite chemical compounds, yet,
as the stoichiometrical laws at present apply chiefly to the latter, all
facts concerning indefinite compounds suffer from inexactitude, and it
is only during recent years that the attention of chemists has been
directed to this province of chemistry.
In chemical mechanics it is, from a qualitative point of view, very im-
portant to clearly distinguish at the very beginning bet ween reversible and
non-reversible reactions. One or several substances capable of reacting on
each other at a certain temperature produce substances which at the same
temperature either can or cannot give back the original substances. For
example, salt dissolves in water at the ordinary temperature, and the
solution so obtained is capable of breaking up at the same temperature,
leaving salt and separating the water by evaporation. Carbon bisul-
phide is formed from sulphur and carbon at the same temperature at
which it can be resolved into sulphur and carbon. Iron, at a certain
temperature, separates hydrogen from water, forming iron oxide, which,
I NTRODUCTION 33
in contact with hydrogen at the same temperature, is able to produce
iron and water. It is evident that if two substances, A and B, give
two others C and D, and the reaction be reversible, then C and D will
form A and B, and, consequently, by taking a definite mass of A,
and B, or a corresponding mass of C and D, we shall obtain, in each
case, all four substances — that is to say, there will be a state of chemical
equilibrium between the reacting substances. By increasing the mass
of one of the substances we obtain a new condition of equilibrium, so
that reversible reactions present a means of studying the influence of
mass on the imnJnx operand* of chemical changes. Many of those
reactions which occur with very complicated compounds or mixtures
may serve as examples of non-reversible reactions. Thus many of the
compound substances of animal and vegetable organisms are broken
up by heat, but cannot be re-formed from their products of decomposi-
tion at any temperature. Gunpowder, as a mixture of sulphur, nitre,
and carbon, on burning, forms gases from which the original substances
cannot be re-formed at any temperature. In order to obtain them, re-
course must be had to an indirect method of combination at the moment
of separation. If A does not under any circumstances combine directly
with B, it does not imply that it cannot give a compound A B. For
A can often combine with C and B with D, and if C has a great
affinity for D, then the reaction of A C on B D produces not only C D,
but also A B. As on the formation of C D, the substances A and B
(previously in A C and B D) are left in a peculiar state of separation,
it is supposed that their mutual combination occurs because they meet
together in this nascent state at the moment of separation (in statu
nascendi). Thus chlorine does not directly combine with charcoal,
graphite, or the diamond, nevertheless there are compounds of chlorine
with carbon and many of them are distinguished by their stability.
They are obtained during the action of chlorine on hydrocarbons, as
the separation products from the direct action of chlorine on hydrogen.
Chlorine takes up the hydrogen, and the freed carbon at the moment
of its separation enters into combination with another portion of the
chlorine, so that in the end the chlorine is combined with both the
hydrogen and the carbon.35
•"•"' Itis possible to imagine that the cause of a great many of such reactions is, that sub-
stances taken in a separate state, for instance, charcoal, present a complex molecule
composed of separate atoms of carbon which are fastened together (united, as is usually
said) by a considerably affinity ; for atoms of the same kind, just like atoms of different
kinds, possess a mutual affinity. The affinity of chlorine for carbon, although unable
to break this bond asunder, may be sufficient to form a stable compound with already
separate atoms of carbon. Such a view of the subject presents a hypothesis which,
although dominant at present, is without sufficiently firm foundation. Were the matter
VOL. I. D
34 PRINCIPLES OF CHEMISTRY
As regards those phenomena which accompany chemical action, the
most important circumstance in reference to chemical mechanics is that
not only do chemical processes produce a mechanical displacement (a
visible disturbance), heat, light, electrical potential and current ; but
that all these agents are themselves capable of changing and governing
chemical transformations. This reciprocity or reversibility naturally
depends on the fact that all the phenomena of nature are only different
kinds and forms of visible and invisible (molecular) movement. First
sound, and then light, was shown to consist of vibratory movements, as
the laws of physics have proved and developed beyond a doubt. Then,
the connection between heat and mechanical motion and work has
ceased to be a supposition, but has become a known fact, and the
mechanical equivalent of heat (424 kilogrammetres of mechanical work
correspond with one kilogram unit of heat or Calorie) gives a mecha.-
nical measure for heat phenomena. Although the mechanical theory
of electrical phenomena cannot be considered so fully developed as the
theory of heat, nevertheless there can be no doubt but that the elec-
trical state of substances, and electric or galvanic currents, represent a
peculiar form of motion ; more especially as both statical and dyna-
mical electricity are produced by mechanical means (in common elec-
trical machines or in Gramme or other dynamos), and, as conversely, a
current (in electric motors) can produce mechanical motion, as heat
produces motion in heat (steam, gas, or air) engines. Thus by passing
a current through the poles of a Gramme dynamo it may be made
to revolve, and, conversely, by revolving it an electrical current is
produced, which demonstrates tlje reversibility of electricity into
mechanical motion. Therefore, chemical mechanics must look for the
fundamental lines of its advancement in the correlation of chemical
with physical and mechanical phenomena. But this subject, owing to
its complexity and comparative novelty, has not yet been subjected to
a harmonious theory, or even to a satisfactory hypothesis, and there-
fore we shall avoid lingering over it.
A chemical change in a certain direction is accomplished not only
as simple as it appears to be, according to this hypothesis, one would expert, for
instance, that the compounds of carbon with chlorine would be easily decomposable by
reason of the supposed considerable affinity of the separate atoms of carbon, which should
therefore tend to mutual combination and the formation of charcoal. It is evident, how-
ever, that not only does reaction itself consist of movements, but that in the compound
formed (in the molecules) the elements (atoms) forming it are in harmonious stable move-
ment (like the planets in the solar system), and this movement will affect the stability
and capacity for reaction, and therefore these depend not only on the affinity of the
participating substances, but also on the conditions of reaction which change the state of
movement of the elements in the molecules, as well as on the nature, form, and inten-
sity of those movements which the elements have in their given state. In a word, the
mechanical side of chemical action must be exceedingly complex.
INTRODUCTION • 35
by reason of the difference of masses, the composition of the sub-
stances concerned, the distribution of their parts, and their affinity or
chemical energy, but also by reason of the conditions under which the
substances occur, and these conditions differ for every particular reac-
tion. In order that a certain chemical reaction may take place between
substances which are capable of reacting on each other, it is often
necessary to have recourse to conditions which are sometimes very
different from those in which the substances usually occur in nature.
For example, not only is the presence of air (oxygen) necessary for the
combustion of charcoal, but the latter must also be heated to redness.
The red-hot portion of the charcoal burns — i.e., combines with the
oxygen of the atmosphere— and in doing so evolves heat, which heats
the adjacent parts of charcoal, which are thus able to burn. Just as
the combustion of charcoal is dependent on its being heated to red-
ness, so also every chemical reaction only takes place under certain
physical, mechanical, or other conditions. The following are the
chief conditions which exert an influence on the progress of chemical
reactions.
(a) Temperature. — Chemical reactions of combination only take
place within certain definite limits of temperature, and cannot be
accomplished outside these limits. As examples we may cite, not only
that the combustion of charcoal begins at a red heat, but also that
chlorine and salt only combine with water at a temperature below 0°.
These compounds cannot be formed at a higher temperature, for they
are then wholly or partially broken up into their component parts.
A certain rise in temperature is necessary to start, combustion. In
certain cases the effect of this rise may be explained as causing one
of the reacting bodies to change from a solid into a liquid or gaseous
form. The transference into a fluid form facilitates the progress of
the reaction, because it aids the intimate contact of the particles acting
on each other. Another reason, to which must be ascribed the chief
influence of heat in exciting chemical action, is that the physical cohe-
sion, or the internal chemical union, of homogeneous particles is thereby
weakened, and therefore the separation of the particles of the sub-
stances taken, and their transference into new compounds, is rendered
easier. When a reaction absorbs heat — as in decomposition, where the
heat is transformed into latent chemical energy — the reason why heat
is necessary is self-evident.
It is most important to observe the effect of an elevation of tem-
perature on all compounds, as there is reason to believe that they are
all decomposed at a more or less high temperature. We have already
seen examples of this in describing the decomposition of mercury oxide
D 2
36 PRINCIPLES OF CHEMISTRY
into mercury and oxygen, and the decomposition of wood under the
influence of heat. Many substances are decomposed at a very mode-
rate temperature ; for instance, the fulminating salt which is employed
in cartridges is decomposed at a little above 120°. The majority of
those compounds which make up the mass of animal and vegetable
matters are decomposed at 250°. On the other hand, there is reason
to think that at a very low temperature no reaction whatever can
take place. Thus plants cease to carry on their chemical processes
during the winter. Every chemical reaction requires certain limits
of temperature for its accomplishment, and, doubtless, many of the
chemical changes observed by us cannot take place in the sun, where
the temperature is very high, or on the moon, where it is very low.
The influence of heat on reversible reactions is particularly instruc-
tive. If, for instance, a compound which is capable of being reproduced
from its products of decomposition be heated up to the temperature at
which decomposition begins, the decomposition of a mass of the sub-
stance contained in a definite volume is not immediately completed.
Only a certain fraction of the substance is decomposed, the other por-
tion remaining unchanged, and if the temperature be raised, the quan-
tity of the substance decomposed increases ; furthermore, for a given
volume the ratio between the part decomposed and the part unaltered
corresponds with each definite rise in temperature until it reaches that
at which the compound is entirely decomposed. This partial decom-
position under the influence of heat is called dissociation. It is pos-
sible to distinguish between the temperatures at which dissociation
begins and ends. Should dissociation proceed at a certain temperature,
yet should the product or products of decomposition not remain in
contact with the still undecomposed portion of the compound, then
decomposition will go on to the end. Thus limestone is decomposed
in a limekiln into lime and carbonic anhydride, because the latter is
carried off by the draught of the furnace. But if a certain mass of
limestone be enclosed in a definite volume — for instance, in a gun
barrel — which is then sealed up, and heated to redness, then, as the
carbonic anhydride cannot escape, a certain proportion only of the
limestone will be decomposed for every increment of heat (rise in tem-
perature) higher than that at which dissociation begins. Decomposition
will cease when the carbonic anhydride evolved presents a maximum
dissociation pressure corresponding with each rise in temperature. If
the pressure be increased by increasing the quantity of gas, then com-"
bination begins afresh ; if the pressure be diminished decomposition
will recommence. Decomposition in this case is exactly similar to
evaporation ; if the steam given off by evaporation cannot escape, its
INTKoIUVTION 37
pressure will reach a maximum corresponding with the given tempera-
ture, and then evaporation will cease. Should steam be added it will
be condensed in the liquid ; if its quantity be diminished — i.e., if the
pressure be lessened, the temperature being constant — then evaporation
will go on. We shall afterwards discuss more fully these phenomena of
dissociation, which were first discovered by Henri St. Claire Deville.
We will only remark that the products of decomposition re-cornbine
with greater facility the nearer their temperature is to that at which
dissociation begins, or, in other words, that the initial temperature of
dissociation is near to the initial temperature of combination.
(b) The influence of an electric current, and of electricity in general,
on the progress of chemical transformations is very similar to the
influence of heat. The majority of compounds which conduct elec-
tricity are decomposed by the action of a galvanic current, and there
being great similarity in the conditions under which decomposition and
combination proceed, combination often proceeds under the influence
of electricity. Electricity, like heat, must be regarded as a peculiar
form of molecular motion, and all that which refers to the influence of
heat also refers to the phenomena produced by the action of an electrical
current, only with this difference, that a substance can be separated
into its component parts with much greater ease by electricity, as the
process goes on at the ordinary temperature. The most stable com-
pounds may be decomposed by this means, and a most important fact
is then observed — namely, that the component parts appear at the
different poles or electrodes by which the current passes through the
substance. Those substances which appear at the positive pole (anode)
-are called ' electro-negative,' and those which appear at the negative
pole (cathode, that in connection with the zinc of an ordinary galvanic
battery) are called 'electro-positive.' The majority of non-metallic
elements, such as chlorine, oxygen, etc., and also acids and substances
analogous to them, belong to the first group, whilst the metals, hydro-
gen, and analogous products of decomposition appear at the negative
pole. Chemistry is indebted to the decomposition of compounds by the
electric current for many most important discoveries. Many elements
have been discovered by this method, the most important being potas-
sium and sodium. Lavoisier and the chemists of his time were not
able to decompose the oxygen compounds of these metals, but Davy
showed that they might be decomposed by an electric current, the
metals sodium and potassium appearing at the negative pole.
(c) Certain unstable compounds are also decomposed by the action of
light. Photography is based on this property in certain substances (for
instance, in the salts of silver). The mechanical energy of those vibra-
38 PRINCIPLES OF CHEMISTRY
tions which determine the phenomena' of light is very small, and there-
fore only certain, and these generally unstable, compounds can be decom-
posed by light — at least under ordinary circumstances. But there is
one class of chemical phenomena dependent on the action of light
which forms as yet an unsolved problem in chemistry — these are the
processes accomplished in plants under the influence of light. Here
there take place most unexpected decompositions and combinations,
which are often unattainable by artificial means. For instance, carbonic
anhydride, which is so stable under the influence of heat and electricity,
is decomposed, and evolves oxygen in plants under the influence of
light. In other cases, light decomposes unstable compounds, such as
are usually easily decomposed by heat and other agents. Chlorine
combines with hydrogen under the influence of light, which shows that
combination, as well as decomposition, can be determined by its action,
as was likewise the case with heat and electricity.
(d) Mechanical effects exert, like the foregoing agents, an action
both on the process of chemical combination and of decomposition.
Many substances are decomposed by friction or by a blow — as, for
example, the compound called iodide of nitrogen (winch is composed of
iodine, nitrogen, and hydrogen), and silver fulminate. Mechanical
friction causes sulphur to burn at the expense of the oxygen contained
in potassium chlorate.
(e) Besides the various conditions which have been enumerated
above, the progress of chemical reactions is accelerated or retarded by
the condition of contact in which the reacting bodies occur. Other
conditions remaining constant, the rate of progress of a chemical re-
action is accelerated by increasing the number of points of contact. It
will be enough to point out the fact that sulphuric acid does not absorb
ethylene under ordinary conditions of contact, but only after con-
tinued shaking, by which means the number of points of contact is
greatly increased. To ensure full action between solids, it is necessary
to reduce them to very fine powder and to mix them as thoroughly as
possible, as by this means their reaction is greatly accelerated. M.
Spring, the Belgian chemist, has shown that finely-powdered solids
which do not react on each other at the ordinary temperature may
undergo reaction under an increased pressure. Thus, under a pressure
of 6,000 atmospheres, sulphur combines with many metals at the ordinary
temperature, and the powders of many inetals form alloys. It is evident
that an increase in the number of points or surfaces must be regarded
as the chief cause producing reaction, which is doubtless accomplished
in solids, as in liquids and gases, in virtue of an internal movement or
mobility of the particles, which movement, although in different degrees
INTKolHXTInN 39
and ton us, must exist in all the states of matter. It is very important
to direct attention to the fact that the internal movement or condition
of the parts of the particles of matter must be different on the surface
of a substance from what it is inside ; because in the interior of a sub-
stance similar particles are acting on all sides of every particle, whilst
at the surface they only act on one side. Therefore, the condition of
a substance at its surfaces of contact with other substances must be
more or less modified by them — it may be in a manner similar to that
caused by an elevation of temperature. These considerations throw
some light on the action in the large class of contact reactions ; that
is, such as seem to proceed from the mere presence (contact) of certain
special substances. Porous or powdery substances are very prone to
act in this way, especially spongy platinum and charcoal. For example,
sulphurous anhydride does not combine directly with oxygen, but this
reaction takes place in the presence of spongy platinum. 36
The above general and introductory chemical conceptions cannot be
thoroughly grasped in their true sense without a knowledge of the
particular facts of chemistry to which we shall now turn our attention.
It was, however, absolutely necessary to become acquainted on the
very threshold with such fundamental principles as the laws of the
indestructibility of matter and of the conservation of energy, as it is
only by their acceptance, and under their direction and influence, that
the examination of particular facts can give practical and fruitful results.
56 Contact phenomena are separately considered in detail in the work of Professor
Konovaloff (1884). In my opinion, one must consider that the state of the internal move-
ments of the atoms in molecules is modified at the points of contact of substances, and
this state determines chemical reactions, and therefore, that reactions of combination,
decomposition, and rearrangement are accomplished by contact. Professor Konovaloff
showed that a number of substances under certain conditions of their surfaces act by con-
tact ; for instance, powdery silica (from the hydrate) acts just like platinum, decom-
posing certain compound ethers. As reactions are only accomplished under close contact,
it is probable that those modifications in the distribution of the atoms in molecules which
come about by contact phenomena prepare the way for them. By this the role of con-
tact phenomena is considerably extended. By such phenomena the fact should be
explained why a mixture of hydrogen and oxygen yields water (explodes) at different
temperatures according to the kind of heated substance which transmits this tempera-
ture. In chemical mechanics, phenomena of this kind have great importance, but as yet
they have been but little studied.
40 PRINCIPLES OF CHEMISTRY
CHAPTER I
ON WATER AND ITS COMPOUNDS
WATER is found almost everywhere in nature, and in all three physical
states. As vapour, water occurs in the atmosphere, and in this form
it is distributed over the entire surface of the earth. The vapour of
water in condensing, by cooling, forms snow, rain, hail, dew, and fog.
One cubic metre (or 1,000,000 cubic centimetres, or 1,000 litres, or
35'316 cubic feet) of air can contain at 0° only 4-8 grams of water, at
20° about 17'0 grams, at 40° about 50*7 grams ; but ordinary air only
contains about 60 per cent, of the possible moisture. Air containing
less than 40 per cent, of the possible moisture is felt to be dry, and air
which contains more than 80 per cent, of the possible moisture is con-
sidered as already damp.1 Water in the liquid state, in falling as rain
1 In practice, the chemist has to continually deal with gases, and gases are often
collected over water; in which case a certain amount of water passes into vapour.
and this vapour mingles with the gases. It is therefore most important that he
should be able to calculate the amount of water or of moisture in <dr and other gasen.
Let us consider the relations in volume and weight which exist in this case. Let us
imagine a cylinder standing in a mercury bath, and filled with a dry gas whose volume
equals u, temperature t°, and pressure or tension li mm. (h millimetres of the column of
.mercury at 0°). We will introduce water into the cylinder in such a quantity that a -mall
part remains in the liquid state, and consequently that the gas will be saturated with
aqueous vapour ; the volume of the gas will then increase (if a larger quantity of water be
taken some of the gas will be dissolved in it, and the volume may therefore be diminished).
We will further suppose that the temperature remains constant after the addition of
the water; then the pressure (as the volume increases the mercury in the cylinder
falls, consequently the pressure is increased) and the volume is increased. In order to
investigate the phenomenon we will artificially increase the pressure, and reduce the
volume to the original volume v. Then the pressure or tension will prove greater than
h, namely h+f, which means that by the introduction of aqueous vapour the tension
of the gas is increased. The researches of Dalton, Gay-Lussac. and Regnatilt showed
that this increase is equal to the maximum pressure which is proper to the aqueous
vapour at the temperature at which the observation is made. The maximum pressure
for all temperatures may be found in the tables made from observations on the tension
of aqueous vapour. The quantity/ will be equal to this maximum pressure of aqueous
vapour. This may be expressed thus : the maximum tension of aqueous vapour land of
all other vapours) saturating a space in a vacuum or in any ,uras U the same. This
rule is known as Dalian's law. Thus we have a volume of dry gas v, under a pressure
h, and a volume of moist gas, saturated with vapour, under a pressure // +/. The volume
v of the dry gas under a pressure h+f occupies, according to the law of Mariotte, a
<»N AVATKK AND ITS COMPOUNDS 41
,-iiul snow, soaks into the soil and collects together into springs, lakes,
livers, seas, and oceans. It is absorbed from the soil by the roots of
volume . ; consequently the volume occupied by the aqueous vapour under the
pre-sure // +/ equals v — -_ , or v* . Thus the volumes of the dry gas and of the
h +f k +/
moisture which occurs in it, at a pressure /*•+/, are in the ratio /: h. And, therefore, if
the aqueous vapour saturates a space at a pressure n, the volumes of the dry air and of
the moisture which is contained in it are in the ratio n—f:f, where / is the pressure of
the vapour according to the tables of vapour tension. Thus, if a volume N of a gas
saturated with moisture be measured at a pressure H, then the volume of the gas, when
TT _ f
dry, will be equal to N • , because the volume N requires to be divided into parts
H
which are in the ratio H— /:/. In fact, the entire volume N must be to the volume of
dry gas x as H is to H-/; therefore, N : x = H : H-/, from which a; = NH~-^. Under
H
TT TT /•
any other pressure — for instance, 760 mm. — the volume of dry gas will be -2:, or ~^
and thus we obtain the following practical rule : If a volume of a gas saturated with
aqueous vapour be measured at a pressure H mm., then the volume of dry gas contained
in it will be obtained by finding the volume corresponding with the pressure H, less the
pressure due to the aqueous vapour at the temperature of observation. For example,
37-5 cubic centimetres of air saturated with aqueous vapour was measured at a tempera-
ture of 15'3°, and under a pressure of 747'3 mm. of mercury (at 0°). What will be the
volume of dry gas at 0° and 760 mm. ? The pressure of aqueous vapour corresponding
witli 15"3C is equal to 12*9 mm., and therefore the volume of dry gas at 15'3° and
747-3 mm. is equal to 37'5 x 747'8~12>y ; at 760 mm. it will be equal to 87'5x Z!£i-
747-3 TOO '
and at 0° the volume of dry gas will be 37'5 x x - — = 34'31 c.c.
760 273-15-3
From this rule may also be calculated what fraction of a volume of gas is occupied by
moisture under the ordinary pressure at different temperatures ; for instance, at 30° C
/=31'5, consequently 100 volumes of a moist gas or air, at 760 mm., contain a volume of
aqueous vapour 100 x >:>1 ;>, or 4'110; also it is found that at 0° there is contained
0'61 p.c. by volume, at 10° 1-21 p.c., at 20° 2'29 p.c.,and at 50° up to 12'11 p.c. From this
it may be judged how great an error might be made in the volumetric determination
of gases were the moisture not taken into consideration. From this it is also evident
how great are the variations in volume of the atmosphere when it loses or gains aqueous
vapour, which again explains a number of atmospheric phenomena (winds, variation of
pressure, precipitations, storms, <fec.).
If aqueous vapour does not saturate a gas, then it is indispensable that the degree of
moisture should be known in order to determine the volume of dry gas from the volume
of moist gas. The preceding ratio gives the maximum quantity of water which can
be held in a gas, and the degree of moisture shows what fraction of this maximum
quantity occurs in a given ease, when the vapour does not saturate the space occupied
by the gas. Consequently, if the degree of moisture equals 50 p.c.— that is, half the
maximum— then the volume of dry gas at 760 mm. is equal to the volume of dry gas
at Till) mm. multiplied by _— -/, or, in general, by ~t' •?, where r is the degree of mois-
ture. It, therefore, it is required to measure the volume of a moist gas, it must either be
entirely dried or quite saturated with moisture, or else the degree of moisture deter-
mined. The first and last methods are inconvenient, and therefore recourse is usually
had to the second. For this purpose water is introduced into the cylinder holding the
gas to be measured ; it is left for a certain time so that the gas may become saturated,
42 PRINCIPLES OF CIIEMJSTRY
plants, which, when fresh, contain from 40 to 80 per cent, of water by
weight. Animals contain about the same amount of water. In a
the precaution being taken that a portion of the water remains in a liquid state; then
the volume of the moist gas is determined, from which that of the dry gas may be
calculated. In order to find the weir/lit <>r' the aq//ca//v ntjixntr in a pis it is necessary
to know the weight of a cubic measure at 0~ and 7(U) mm. Knowing that one cubic
centimetre of air under these circumstances weighs O'OOl'J'.Ki gram, and that the density
of aqueous vapour is 0'62, we find that one cubic centimetre of aqueous vapour at 0° and
760 mm. weighs 0'0008 gram, and at a temperature t and pressure // the weight of one
cubic centimetre will be O'OOOS x — x — ^ -- . We already know that v volumes of a ga^
at a temperature t° pressure h contain v x •-- volumes of aqueous vapour which satu-
rate it, therefore the weight of the aqueous vapour held in v volumes of a gas will bt
VJ1/ x 0-0008 x Ax -™ , or z; x O'OOOS x f x >27l! .
h 7GO 273 + t° 7(50 878 + <
Consequently, the weight of the water which is held in one volume of a gas is only
dependent on the temperature and not on the pressure. This also signifies that evapo-
ration proceeds to an equal extent in air as in a vacuum, or, in general terms (this is
Dalian's law), vapours and gases diffuse into each other as if into a vacuum. In a given
space there enters, at a given temperature, a constant quantity of vapour whatever be
the pressure of the gas filling that space. If the degree of moisture equals r then the
weight of the vapour in v cubic centimetres will be y) = v x O'OOOS x J[ x J ' grams,
7oO
From this it is clear that if the weight of the vapour held in a given volume of a gas
be known, it is easy to determine the degree of moisture r= u-ono X / X •)-•>'
On this is founded the very exact determination of the degree of moisture of air by the
weight of water contained in a given volume. It is easy to calculate from the preceding
formula the number of grams of water contained at all pressures in one cubic metre or
million centimetres of air saturated with vapour at various temperatures ; for example,
at 80° /= 31-5, therefore p = 1000000 x 0*0008 x ^ x 27g + g(j or 2<)\S4 grams.
The laws of "Mariotte, Dalton, and Gay-Lussac, which are here applied to gases and
vapours, are not entirely exact, but are approximately true. Were they unite exact, a mix-
ture of several liquids, having a certain vapour pressure, would be able to give vapours
of a very great pressure, which is not the case. In fact the pressure of aqueous vapour
is slightly less in a gas than in a vacuum, and the weight of aqueous vapour held in a
gas is slightly less than it should be according to Daltoifs law. as was shown by the ex-
periments of Eegnault and others. This means that the tension of the vapour is less
in air than in a vacuum, which also is the reason why the weight of vapour is less than
the theoretical weight. The difference between the pressure of vapours in air and in a
vacuum does not, however, exceed ^ of the total pressure of the vapours, and therefore
in practice the application of Dalton's law may be followed. This i/rcm/trnf in rajtour
tension which occurs in the intermixture of vapours and gases, although small, indicates
that there is then already, so to speak, a beginning of chemical change. The essence of
the matter is that in this case there occurs as on contact (see preceding footnote) an
alteration in the movements of the atoms in the molecules, and therefore also a change
in the movement of the molecules themselves.!
In the uniform intermixture of air and other gases with aqueous vapour, and in tin-
capacity of water to pass into vapour and form a uniform mixture with air, we may
perceive an instance of a physical phenomenon which is analogous to chemical phe-
nomena, forming indeed a transition from one class of phenomena to the other. Between
water and dry air there exists a kind of affinity which obliges the water to saturate the
<)N WATKIi AND ITS COMPOUNDS 43
solid state water appears ;is snow, ice, or in an intermediate form
lit -tween these two, which is seen on mountains covered with perpetual
sn<i\\r. The water of rivers,- springs, oceans and seas, lakes, and wells
air. But such a homogeneous mixture is formed (almost) independently of the nature of
the pis in which evaporation takes place; even in a vacuum the phenomenon occurs in
exactly the same way as in a pis, and therefore it is not the property of the gas, nor its
relation to water, but the property of the water itself, which obliges it to evaporate, and
therefore in this case chemical affinity is not yet acting — at least its action is not clearly
pronounced. That it does, however, play a certain part is seen from the deviation from
Dalton's law.
- In falling through the atmosphere, water dissolves the gases of the atmosphere,
nitric acid, ammonia, organic compounds, salts of sodium, magnesium, and calcium, and
mechanically washes out a mixture of dust and microbes which are suspended in the
atmosphere. The amount of these and certain other constituents is very variable. Even
in the beginning and end of the same rainfall, a variation which is often very considerable
may be remarked. Thus, for example, Bunsen found that rain collected at the begin-
ning of a shower contained 3'7 grams of ammonia per cubic metre, whilst that collected
at the end of the same shower contained only 0'64 gram. The water of the entire
shower contained an average of 1*47 grams of ammonia per cubic metre. In the course
of a year rain supplies an acre of ground with up to 5^ kilos of nitrogen in a combined
form. Marchand found in one cubic metre of snow water 15'03, and in one cubic metre
of rain water 10'07, grams of sodium sulphate. Angus Smith showed that after a thirty-
hours' fall at Manchester the rain still contained 34'3 grams of salts per cubic metre. A
considerable amount of organic matter, namely 25 grams per cubic metre, has been found
in rain water. The total amount of solid matter in rain water reaches 50 grams per
cubic metre. Rain water contains generally very little carbonic acid, whilst stream
water contains a considerable quantity of it. In considering the nourishment of
plants, it is necessary to keep in view the substances which are carried into the soil
by rain.
River ivater, which is accumulated from springs and sources fed by atmospheric
water, contains from 50 to 1,600 parts by weight of salts in 1,000,000 parts. The amount
of solid matter, per 1,000,000 parts by weight, contained in the chief rivers is as
follows :— the Don 124, the Loire 135, the St. Lawrence 170, the Rhone 182, the Dnieper
187, the Danube from 117 to 234, the Rhine from 158 to 317, the Seine from 190 to 432,
the Thames at London from 400 to 450, in its upper parts 387, and in its lower parts up to
1,017, the Nile 1,580, the Jordan 1,052. The Neva is characterised by the remarkably
small amount of solid matter it contains. From the investigations of Prof. G. K. Trapp,
a cubic metre of Neva water contains 32 grams of incombustible and 23 grams of
organic matter, or altogether about 55 grams. This is one of the purest waters which is
known in rivers. The large amount of impurities in river water, and especially of organic
impurity produced by pollution with putrid matter, makes the water of many rivers unfit
for n-.e.
The chief part of the soluble substances in river water consists of the calcium salts.
100 parts of the solid residues contain the following amounts of calcium carbonate —
from the water of the Loire 53, from the Thames about 50, the Elbe 55, the Vistula 65,
the Danube 05, the Rhine from 55 to 75, the Seine 75, the Rhone from 82 to 94. The
Neva contains 40 parts of calcium carbonate per 100 parts of saline matter. The con-
siderable amount of calcium carbonate held by stream water is very easily explained from
the fact that water which contains carbonic acid in solution easily dissolves calcium
carbonate, which occurs all over the earth. Besides calcium carbonate and sulphate,
river water contains magnesium, silica, chlorine, sodium, potassium, aluminium, nitric acid,
and manganese. The presence of salts of phosphoric acid has not yet been determined
with exactitude for all rivers, but the presence of nitrates has been proved with certainty
in almost all kinds of well-investigated river water. The quantity of calcium phosphate
does not exceed 0'4 gram in the river of the Dnieper, and the Don does not contain more
44 PHIXCIL'LES OF CHEMISTRY
contains various substances in solution, mostly salts —that is, sub-
stances resembling common table salt in their physical properties and
than 5 grams. The water of the Seine contains about 15 grams of nitrates, and the Rhone
about 8 grams. The amount of ammonia is much less ; thus in the water of the Rhine
about 0*5 gram in June, and 0'2 gram in October ; the water of * he Seine contains the
same amount. This is less than in rain water. Notwithstanding this insignificant
quantity, the water of the Rhine alone, which is not so very large a river, carries U'>.'_!4.1
kilograms of ammonia into the ocean every day. The difference between the amount oi
ammonia in rain and river water depends on the fact that the soil through which tht
rain water passes is able to withhold the ammonia. (Soil can also absorb many othei
substances, such as phosphoric acid, potassium salts, Arc.)
The water of springs, rivers, wells, and in general of those localities from which it is
taken for drinking purposes, may be very injurious to the health if it contains much
organic pollution — all the more, as in such water the lower organisms (bacteria) maj
rapidly develop, and these organisms often serve as the carriers or causes of infectious-
diseases. Thanks to the work of Pasteur, Koch, and many others, this province of researcl
has made considerable progress during the past ten years, and has shown the possi-
bility of investigating even the number and properties of the germs held by water
because those pathogenic bacteria which produce sickness, such as typhoid fever, Siberiai
plague, &c., have been distinguished. In bacteriological researches, a gelatinous
medium, enabling the germs to develop and multiply, is prepared with gelatin and water
which has previously been heated several times, at intervals, to 100° (it is thus renderec
sterile — that is to say, all the germs in it are killed). The water to be investigated
is added to this prepared medium in a definite and small quantity (it is sometimes
diluted with sterilised water to facilitate the calculation of the number of germs), it is
protected from dust (which contains germs), and is left at rest until whole families o:
lower organisms are developed from each germ. These families (colonies) are visible tc
the naked eye (as spots), they may be counted, and by examining them under th<
microscope and observing the number of organisms they produce, their significance ma>
be determined. The majority of bacteria are harmless, but there decidedly are patho
genie bacteria whose presence is one of the causes of malady, and of the spreading o
certain diseases. The number of bacteria in one cubic centimetre of water sometime!
attains the immense figures of hundreds of thousands and millions. Certain well, spring
and river waters contain very few bacteria, and are free from disease-producing bacterii
under ordinary circumstances. By boiling water, the bacteria in it are killed, but th<
organic matter necessary for their nourishment remains in the water. The best kind:
of water for drinking purposes do not contain more than 800 bacteria in a cnbii
centimetre.
The presence in water of every residue of destroyed organisms may be partly judgc<
from the amount of combined nitrogen, as all organisms contain nitrogen compotmdfl
It is mo'st essential to distinguish and determine nitrogen in the form of organic mattei
and in the form of oxides (nitric acid). The former is not separated, on heating, iron
water by the action of reducing agents, such as sulphurous anyhdride, whilst thi
nitrogen which occurs as oxide is evolved by this means. Thus on adding hydrochlori
"acid and ferrous chloride to water, the nitrogen of the nitric acid gives oxide of nitrogen
which may be determined. The presence of nitric acid indicates that the organr
matter in water has already been oxidised. Water which contains more than 1 par
of nitrogen (in this form) in a million parts is considered as injurious, and should no
be used. Frankland found about r.s parts of nitrogen in an oxidised form, and Iron
0'22 to 0*5 part in organic combinations in the water of the Thames at London.
The amount of gases dissolved in river water is much more constant tha
solid constituents. One litre, or 1,000 c.c., of water contains 40 to 5,1
measured at normal temperature arid pressure. In winter the amount of <ra
than in summer or autumn. Allowing that a litre contains 50 c.c. of gases, it may b
admitted that these consist, on the average, of 20 vols. of nitrogen, 20 vols. of carl ion i
ON AY.V
AND ITS roMI'<TNI).«
45
cliicf
chemical transformations. Further, the quantity and nature of
salts differ in different waters.3 Everybody knows that there
anhydride
of 10 vols.
still in abc
dominates
which .sin
succeeded,
anhydride,
Deville. CO
litre. Fir
M'oeeeding in all likelihood from the soil and not from the atmosphere), and
if oxygen. If the total amount of gases be less, the constituent gases are
h
>ut the same proportion; in many ca>es, however, carbonic anhydride pre-
The water of many deep and rapid rivers contain?-, less carbonic anhydride,
\s their rapid formation from atmospheric water and that they have not
during a long and slow course, in absorbing a greater quantity of carbonic
Thus, for instance, the water of the Khine, near Strasburg, according to
itains M c.c. of carbonic anhydride, 16 c.c. of nitrogen, and 7 c.c. of oxygen per
n the researches of Prof. M. R. Kapoustin and his pupils, it appears that in
determining the quality of a water for drinking purposes, it is most important to investi-
gate the composition of the dissolved gases.
3 Sprinij water is formed from rain water percolating through the soil. Naturally a
part of the rain water is evaporated straightway from the surface of the earth and from
the vegetation on it. It has been shown that out of 100 parts of water falling 011 the
earth only 36 parts flow to the ocean ; the remaining 64 are evaporated, or percolate
far underground. The collection of water by means of ponds, common wells, or artesian
wells is dependent on the presence of subterranean water. After flowing underground
along some impervious strata, water comes out at the surface in many places as springs,
whose temperature is determined by the depth from which the water has flowed.
Springs penetrating to a great depth may become considerably heated, and this is why
hot mineral springs, with a temperature of up to 30° and higher, are often met with. For
instance, there is one Caucasian spring whose temperature is 90°. Most likely in this
Ban the water is heated owing to its penetrating near a rock formation which is heated
by volcanic action. The composition of spring water is most varied. When a spring
water contains substances which endow it with a peculiar taste, and especially if these
substances are such as are only found in minute quantities or not at all in river and
other flowing waters, then the spring water is termed a mineral water. Many such
waters are employed for medicinal purposes. Mineral waters are classed according to
their composition into — (a) saline waters, which often contain a large amount of common
salt; (b) alkaline waters, which contain sodium carbonate; (c) bitter waters, which
contain magnesia ; (d) chalybeate waters, which hold iron carbonate in solution ; (e}
aerated waters, which are rich in carbonic anhydride ; ( f ) sulphuretted waters, which
contain hydrogen sulphide. Sulphuretted waters may be recognised by their smell of
rotten eggs, and by their giving a black precipitate with lead salts, and also by their tar-
nishing silver objects. Aerated waters, which contain an excess of carbonic anhydride,
effervesce in the air, have a sharp taste, and redden litmus paper. Saline waters leave a
large residue of soluble solid matter on evaporation, and have a salt taste. Chalybeate
4
z2
9 G^
j
-_
3
3
•f € 1 g 1 .2
- I
— 5
||
P 03 & 53 <"3 i O 'S
|| "I « "I * ™
§1
'55 «j
5
ii
it
wf =11
°
|g
^ 0
1^
fj S-s "as* ^ §
a
O ™
1 "5 «g~ , H
I.
1,928
152
24
448
152 1,300 80 1 2,609
II.
816
386
1,239
26 i —
43
9
257
46 1,485 I ! 2,812
III.
1,085
1,430
1,105
4
90
—
187
05 1,326 11 j 3,950
IV.
343
3,783
16
3,431 —
14
—
251
112 2,883 —
V. 3,406
15,049
- 2
—
17
1,587
229 76 20,290
VI. 352
3,145
—
€5 35
50
1
260
11 iu — 3,970
VII. 30K
1,036
2,583 1,261 , —
—
4
178
75 ' _ 1 5 451
VIII.1 1.7LV,
9,480
—
— | 40
120
26
208
40 — 11*790
IX. 551
2,040
1,150
999 , —
1
30
209
50 2,740 i 4,070
X. 285
558
279
3,813 1 —
— 7
45
45 2,268 1 5,031
XL 340
910
Iron and aluminium sulphates : | {'ggn
940
190 2 550 ( SulPhuric
'DO/. ] aild hydro-
( chloric acids
46 PRINCIPLES OK CIIK.M1STKY
are salt, fresh, iron, and other waters. The presence of about 3^ per
cent, of salts renders sea-water 4 heavy and bitter to the taste. Fresh
water also contains salts, only in a comparatively small quantity.
Their presence may be easily proved by simply evaporating water in a
vessel. By evaporation the water passes away as vapour, whilst the
salts are left behind. This is why a crust (incrustation), consisting of
salts, previously in solution, is deposited on the insides of kettles or
boilers, and other vessels in which water is boiled. Running water
(rivers, etc.) is charged with salts, owing to its being formed from the
collection of rain water percolating through the soil. While percolating
the water dissolves certain parts of the soil. Thus water which niters
or passes through saline or calcareous soils becomes charged with salts
or contains calcium carbonate (chalk). Rain water and snow are much
purer than river or spring water. This is because snow and rain are
only condensed aqueous vapour, and salts do not pass into the vapour.
waters have an inky taste, and are coloured black by an infusion of galls ; on being
exposed to the air they usually give a brown precipitate. Generally, the character of
mineral waters is mixed. In the table on page 45 are given the analysis of certain
mineral springs which are known for their medicinal properties. The quantity of the
substances is expressed in millionths by weight — that is, in grams per cub. metre or
milligrams per litre.
I. Sergieffsky, a sulphur water, Gov. of Samara (temp. 8° C.), analysis by Clause.
II. Geleznovodskya water source No. 10, near Patigorsk, Caucasus (temp. 22'5°), analysis
by Fritzsche. III. Aleksandroff sky, alkaline-sulphur source, Patigorsk (temp. 46'5°), average
of analyses by Herman Zinin and Fritzsche. IV. Bougountouksky, alkaline source,
No. 17, Essentoukah, Caucasus (temp. 21'6°), analysis by Fritzsche. V. Saline water,
Staro-Russi, Gov. of Novgorod, analysis by Nelubin. VI. Water from artesian well at
the factory of state papers, St. Petersburg, analysis by Struve. VII. Spriidel, Carlsbad
(temp. 83'7°), analysis by Berzelius. VIII. Kriitznach spring (Elisenquelle), Prussia
(temp. 8'8°), analysis by Bauer. IX. Eau de Seltz, Nassau, analysis by Henry. X. Vichy
water, France, analysis by Berthier and Puvy. XI. Paramo de Ruiz, New Granada,
analysis by Levy ; it is distinguished by the amount of free acids.
4 Sea-water contains more non-volatile saline constituents than the usual kinds of
fresh water. This is explained by the fact that the waters flowing into the sea supply
it with salts, and whilst a large quantity of vapour is given off from the surface of the
sea, the salts remain behind. Even the specific gravity of sea-water differs con-
siderably from that of pure water. It is generally about T02, but in this and also in
respect to the amount of salts contained, samples of sea-water from different localities
and from different depths offer rather remarkable variations. It will be sufficient to
point out that one cubic metre of water from the undermentioned localities contains the
following quantity in grams of solid constituents :— Gulf of Venice 19,1^2, L«-gli..rn
Harbour 24,812, Mediterranean, near Cetta, 87,655, the Atlantic Ocean from :j-2.:,sr, t.«,
85 695 the Pacific Ocean from 85,283 to 84,708. In closed s'eas which do not communi-
cate, or are in very distant communication, with the ocean, the difference is often still
greater. Thus the Caspian Sea contains 6,800 grams ; the Black Sea and Baltic 17,700.
Common salt forms the chief constituent of the saline matter of sea- or ocean-water ; thus
in one cubic metre of sea-water there are 25,000-81,000 grams of common salt, '2,C,(>0-
6,000 grams of magnesium chloride, 1,200-7,000 grams of magnesium sulphate, i.:,oo-t;,<H)i>
grams of calcium sulphate, and 10-700 grams of potassium chloride. The small amount
of organic matter and of the salts of phosphoric acid in sea- water is very remarkable.
ON AVATKR AND ITS COMPOUNDS 47
Neverthrlos. in passing through the atmosphere, r;i in and snow succeed
in catcliinu' tin- .lust held in it, and dissolve air, which is found in every
water. The dissolved gases of the atmosphere are partly disengaged,
as bubbles from water on heating, and water after long boiling is quite
freed from them.
In general terms water is called pure when it is clear and free from
insoluble particles held in suspension and visible to the naked eye, from
which it may be freed by nitration through charcoal, sand, or porous
(natural or artificial) stones, and when it possesses a clean fresh taste.
It depends on the absence of any tastable, decomposing organic matter,
on the quantity of air 5 and atmospheric gases in solution, and on the
presence of mineral substances to the amount of about 300 grams per
ton (or cubic metre, or, what is the same, 300 milligrams to a kilo-
gram or litre of water), and of not more than 100 grams of organic
matter.6 Such water is suitable for drinking and every practical
5 The taste of water is greatly dependent on the quantity of dissolved gases it con-
tains. On boiling, these gases are given off, and it is well known that, even when cooled,
boiled water has, until it has succeeded in absorbing gaseous substances from the atmo-
sphere, quite a different taste from fresh water containing a considerable amount of gas.
The dissolved gases, especially oxygen and carbonic anhydride, have an important
influence on the health. The following instance is very instructive in this respect. The
Grenelle artesian well at Paris, at the first period of its opening, supplied a water which
had an injurious effect on animals and people. It appeared that this water did not
contain oxygen, and in general was very poor in gases. As soon as it was made to fall in
a cascade, by which it absorbed air, it proved entirely fit for consumption. In long sea
voyages by steamer sometimes fresh water is not taken or only taken in a small quantity
because it spoils by keeping, and becomes putrid from the organic matter it contains under-
going decomposition. Fresh water may be obtained directly from sea-water by distilla-
tion. The distilled water 116 longer contains sea salts, and is therefore fit for consump-
tion, but it is very tasteless and has the properties of boiled water. In order to render it
palatable certain salts, which are usually held in fresh water, are added to it, and it is
made to flow in thin streams exposed to the air in order that it may become saturated
with the component parts of the atmosphere — that is, absorb gases.
6 Hard icat^r is such as contains much mineral matter, and especially a large pro-
portion of calcium salts. Such water, owing to the amount of lime it contains, does not
form a lather with soap, prevents vegetables boiled in it from softening properly, and
forms a great deal of incrustation on vessels in which it is boiled. Owing to its high
degree of hardness, it is injurious for drinking purposes, which is evident from the fact
that in many large cities the death-rate decreased after introducing a soft water in the
place of a hard water. Putrid water contains a considerable quantity of decomposing
organic matter, chiefly vegetable, but in populated districts, especially in towns, chiefly
animal remains. Such water acquires an unpleasant smell and taste, by which stagnant
bog water and the water of certain wells in inhabited districts are particularly charac-
terised. Such water is especially harmful at a period of epidemic. It may be partially
purified by passing through charcoal, which retains the putrid and certain organic sub-
stances, and also certain mineral substances. Turbid water may be purified to a certain
extent by the addition of alum, which aids, after standing some time, the formation of a
sediment. Condy's fluid (potassium permanganate) is another means for purifying
putrid water. A solution of this substance, even if very diluted, is of a red colour ; on
adding it to a putrid water, the permanganate oxidises and destroys the organic matter.
When added to water in such ;i quantity as to impart to it an almost imperceptible rose
48
PRINCIPLES OF CHEMISTRY
application, but evidently it is not pure in a chemical sense. A
chemically pure water is necessary not only for scientific purposes, as
an independent substance having constant and definite properties, and
as the chief component of all forms of water which play such an impor-
tant part in nature, but also for many practical purposes — for instance,
in photography and in the preparation of medicines — because many
properties of substances in solution are changed by the impurities of
natural waters. Water is usually purified by distillation, because the
solid substances in solution are not transformed into vapours in this
process. Such distilled water is prepared by chemists and in labora-
tories by boiling water in closed metallic boilers or stills, and causing
the steam produced to pass into a condenser — that is, through tubes
(which should be made of tin, or, at all events, tinned, as water and its
impurities do not act on tin) surrounded by cold water, and in which
the steam, being cooled, condenses into water which is collected7 in a
colour it destroys much of the organic substances it contains. It is especially salutary
to add a small quantity of Condy's fluid to impure water in times of epidemic.
The presence in water of one gram per litre, or 1,000 grams per cubic metre, of any
substance whatsoever renders it unfit and even injurious for consumption by animals,
and this whether organic or mineral matter predominate. The presence of 1 p.c. of
chlorides makes water quite salt, and produces thirst instead of assuaging it. The
presence of magnesium salts is most unpleasant ; they have a disagreeable bitter taste,
and in fact impart to sea water its peculiar taste. A large amount of nitrates is only
found in impure water, and is usually injurious, as they may indicate the presence of
decomposing organic matter.
7 Distilled water may be prepared, or distillation in general carried on, either in a
FIG. 4.— Distillation by means of a metallic still. The liquid in C is heated by the fire F. The
vapours rise through the head A and pass by the tube T to the worm S placed in a vessel R,
through which a current of cold water flows by means of the tubes D and P.
o.\ WATKi; AND ITS COMPOUNDS
1!)
receiver. By standing exposed to the atmosphere, however, the water
in time absorbs air, and dust carried in the air, and ceases to be en-
tirely pure. However, the amount of impurities in distilled water is
so small that they have hardly any effect on the properties of the
water, and it is fit for many purposes. Nevertheless, in distillation,
water retains, besides air, a certain quantity of volatile impurities
(especially organic) and the walls of the distillation apparatus are
partly corroded by the water, and a portion, although small, of their
substance renders the water not entirely pure, thus a sediment is ob-
tained on evaporation'.8
Still, for certain physical and chemical researches it is necessary to
have completely pure water. To obtain it a solution of potassium
permanganate is added to distilled water until it all becomes tinted
light rose colour. By this means the organic matter in the water is
destroyed (converted into gases or non-volatile substances). An excess
metal still with worm condenser (fig. 4), or on a small scale in the laboratory in a glass
retort (fig. 5) heated by a lamp (see footnote 19, Introduction). Fig. 5 illustrates
the main parts of the usual glass laboratory apparatus used for distillation. The steam
FIG. 5. — Distillation from a glass retort. The neak of the retort fits into the inner tube of the
Ltebig's condenser. The space between the inner and outer tube of the condenser is filled with
col< I water, which enters by tliu tube g and Hows out at/.
issuing from the retort (on the right-hand side) passes through a glass tube surrounded
by a larger tube, through which a stream of cold water passes, by which the steam is
cuii.lcnsed and trickles into a receiver (on the left-hand side). •
8 One of Lavoisier's first memoirs (1770) referred to this question. He investigated
the formation of the earthy residues in the distillation of water in order to prove whether
it was possible, as was affirmed, to convert water into earth, and he found that the
residue was produced by the action of water on the walls of the vessel holding it, a:id
not from the water itself. He proved this to be the case by direct weighing.
VOL. I. E
50 PRINCIPLES OK CHEMISTRY
of potassium permanganate does no harm, because in the next distilla-
tion it is left behind in the distillation apparatus. The next distilla-
tion should then be from a platinum retort with a platinum receiver.
Platinum is a metal which is not in any way changed either by air or
water, and therefore nothing passes from it into the water. The water
obtained in the receiver still contains air. It must then be boiled for
a long time, and afterwards cooled in a vacuum under the receiver
of an air pump. Pure water on evaporation does not give any sedi-
ment, does not in the least change, however long it be kept, and if air
have no access to it does not putrefy like water only once distilled or
impure ; and it does not give bubbles of gas on heating, nor does it
change the colour of a solution of potassium permanganate. These
are a few signs by which the complete purity of water may be recog-
nised.
Water, purified as above described, has constant physical and
chemical properties. For instance, it is of such water only that one
cubic centimetre weighs one gram at 4° C. — -i.e., it is only such pure
water whose specific gravity equals 1 at 4° C.9 Water in a solid state
forms crystals of the hexagonal system10 which are seen in snow, which
9 Taking the generally-accepted specific gravity of water at its greatest density — i.e.
at 4° as 1 — it has been shown by experiment that the specific gravity of water at different
temperatures is as follows : —
At - 5D . . . 0-99929 At 30° ... G'99577
., 0° ... 0-9JHIS7 „ 40° . . . 0-99230
,,4-10° . . . 0-99974 „ 50° . . . 0'98817
„ 15° ... 0-9991C) „ 80° ... 0-97192
20° 0*99820 100: 0-9.VO4
Water at 4° is taken as the basis for reducing measures of length to measures of
weight and volume. The metric, decimal, si/stem of measures of weights and volumes is
generally employed in science. The starting point of this system is the metre (39'37
inches) divided into decimetres ( = 0'1 metre), centimetres ( = O'Ol metre), millimetres
( = O'OOl metre), and micrometres (- one millionth of a metre). A cubic decimeti'e is
called a litre, and is used for the measurement of volumes. The weight of a litre of
water at 4° in a vacuum, is called a kilogram. One thousandth part of a kilogram, or one
cubic centimetre, of water weighs one yratn. It is divided into decigrams, centigrams,
and milligrams ( = O'OOl gram). An English pound equals 453'59 grams. The great
advantage of this system is that it is a decimal one, and that it is universally adopted in
science and in most international relations. All the mecuures died in thin ii-orl; (in-
metrical. The units most often used in science are : — Of length, the centimetre ; of
weight, the gram ; of time, the second ; of temperature, the degree Celsius or Centigrade.
10 As solid substances appear in independent, regular, crystalline forms which are
dependent, judging from their cleavage or lamination (in virtue of which mica breaks
up into laminae and Iceland spar, &c., into pieces bounded by faces inclined to each other
at angles which are definite for each substance), on an inequality of attraction (cohesion
hardness) in different' .'directions which intersect at definite angles ; therefore, the
determination of crystalline forms offers one of the most important external marks
<>N WATF.U AND ITS COMPOUNDS
51
c<>M>isrs < »f star-like clusters of several crystals, and also in
tin- half-incited scattered ice floating on rivers in spring time. At
characterising separate, definite chemical compounds. The elements of crystallography
\vhi«-h comprise a -)>• •< -i:il science, sh mid therefore be familiar to all who desire to work
Fi<;. r,. — Example <>t rhc form belonging to the FIG. 7. — Rhombic Dodecahedron of the regular
regular system. Combination of an octahedron system. Garnet.
an< I a cube. The former predominates. Alum,
thior spar, suboxide of copper, ami others.
s. Hexagonal prism ti-nninati-d by hexagonal Fio. 9.— Rhombohedron. Ca!c spar,
>. Quaitz. &c.
Fa;. 10.— lihnmbic system. FK;. 11. — Triclinia pyramid.
Desminc.
FIG. 12. — Triclinic sv.-tt in.
AU.ite, «fcc.
in scientific chemistiy. In this work we shall only have occasion to speak of a few
crystalline forms, some of which are shown in Figs. 6 to 12.
E —
52 PRINCIPLES OF CHEMISTRY
this time of the year the ice splits up into spars or prisms, bounded by
angles proper to substances crystallising in the hexagonal system. The
temperatures at which water passes from one state to another are
taken as fixed points on the thermometer scale : namely, the zero
corresponds with the temperature of melting ice, and the temperature
of the steam disengaged from water boiling at the normal barometer
pressure (that is 760 millimetres measured at 0°, at the latitude of 45°,
at the sea level) is taken as 100° of the Celsius scale. Thus, the fact
that water liquefies at 0° and boils at 100° is taken as one of its
properties as a definite chemical compound. The weight of one cubic
metre of water at 4° is 1,000 kilos, at 0° it is 999'8 kilos. The weight
of a cubic metre of ice at 0° is less — namely, 917 kilos ; the weight of a
cubic metre of water vapour at 760 mm. pressure and 100° is only 0'60
kilos ; the density of the vapour compared with air =; 0'62. and com-
pared with hydrogen = 9.
These data briefly enumerate the physical properties of water as a
separate substance. As a supplement to this it may be added that water
is a mobile liquid, colourless, transparent, without taste or smell, £c.
It is unnecessary to dwell on these properties here, as water is familiar
to all ; other properties will also be pointed out in describing less known
substances. Its latent heat of vaporisation is 534 units, of liquefac-
tion 79 units of heat.11 The large amount of heat stored up in water
11 Of all known liquids, water exhibits the greatest cohesion of particles. Indeed, it
ascends to a greater height in capillary tubes than other liquids ; for instance, two and a
half times as high as alcohol, nearly three times as high as ether, and to a much greater
height than oil of vitriol, &c. In a tube of two millimetres diameter, water at 0° ascends
15 '8 millimetres, counting from the level of the liquid to two-thirds of the height of the
meniscus, and at 100° it rises 12'5 millimetres. The cohesion varies very uniformly with
the temperature ; thus at 50° the height of the capillary column equals 13'i) millimetres —
that is, the mean between the columns at 0° and 100°. This uniformity is not destroyed
even on approaching the freezing point, and gives reason to think that at high tempera-
tures cohesion will vary as uniformly as at ordinary temperatures ; that is, the difference
between the columns at 0° and 100° being 2'8 millimetres, the height of the column at
500° should be 15;/- (5 x 2'8) = Vji millimetres. Consequently, at these high temperatures
the cohesion between the particles of water would be almost nil. Only certain solutions
(sal ammoniac r.i-a lithium chloride), and these only with a great excess of water, rise
higher than pure water in capillary tubes. The great cohesion of water doubtless
determines many of both its physical and chemical properties.
The quantity of heat required to raise the temperature of one part by weight of(
water from 0° to 1°, i.e., by 1° C., is called the unit of heat or calorie; the specific
heat of liquid water at 0° is taken as equal to ttnity. The variation of this specific
heat with a rise in temperature is inconsiderable in comparison with the variation
exhibited by the specific heats of other liquids. According to Ettinger, the specific heat
of water at 20° =1'016, at 50° = r039, and at 100° = 1'078. The specific heat of water is
greater than that of all other known liquids ; for example, the specific heat of alcohol at
0° is 0'5475 — i.e., the quantity of heat which raises 55 parts of water 1° raises 100 parts
of alcohol 1°. The specific heat of oil of turpentine at 0° is 0'4106, of ctli.-r <f,V2<), of
acetic acid G'527-4, of mercury 0'038. This means that water is the best condenser or
ON WAT Hi; AND ITS COMPOUNDS 53
vapour and also in liquid water (for its specific heat is greater than
that of other liquids) renders it available in both forms for heating
absorber of heat. This property of water has an important significance in practice and
in nature. Water impedes rapid cooling or heating ; it tempers cold and heat. The
specific heats of ice and aqueous vapour are much less than that of water ; namely
that of ice is 0'504, and of steam 0'48.
With an irerease in pressure equal to one atmosphere, the compressibility of water is
0-000047, of mercury 0-00(KHI:!.VJ, of ether 0'00012 at 0°, of alcohol at 13° O'QO.0095. The
addition of various substances to water generally simultaneously decreases its com-
pressibility and cohesion. The compressibility of other liquids increases with a rise of
temperature, but for water it decreases up to 53° and then increases like other liquids.
The expansion of /rate?- by heat (Note 9) also exhibits many peculiarities which are
not found in other liquids. The expansion of water at low temperatures is very small
compared with other liquids ; at 4° it reaches even 0, and at 100° it is equal to O'OOOS ;
below 4° it is negative — i.e., water on cooling then expands, and does not decrease in
volume. In passing into a solid state, the specific gravity of water decreases ; at 0° one
c.c. of water weighs 0-999888 gram, and one c.c. of ice at the same temperature weighs only
0'9175 gram. The ice formed, however, contracts on cooling like the majority of other
substances. Thus 100 volumes of ice are produced from 92 volumes of water — that is,
water expands considerably on freezing, which fact determines a number of natural
phenomena. The freezing point of water falls with an increase in pressure (0'007J per
atmosphere), because in freezing water expands (Thomson), whilst with substances which
contract in solidifying the melting point rises with an increase in pressure ; thus, for
paraffin it is at one atmosphere 46° and at 100 atmospheres 49°.
When liquid water passes into vapour, the cohesion of its particles must be destroyed,
as the particles are removed to such a distance from each other that their mutual
attraction no longer exhibits any influence. As the cohesion of aqueous particles varies at
different temperatures, the quantity of heat which is expended in overcoming this
cohesion — or the latent heat of evaporation — for this reason alone will be different at
different temperatures. The quantity of heat which is consumed in the transformation
of one part by weight of water, at different temperatures, into vapour was determined by
Regnault with great accuracy. His researches showed that one part by weight of water
taken at 0°, in passing into vapour having a temperature t°, consumes 606'5 + 0'305£ units
of heat, at 50° (521-7, at 100° 637'0, at 150 652'2, and at 200° 667'5. But this
quantity includes also the quantity of heat required for heating the water from 0° to t° —
i.e., besides the latent heat of evaporation, also that heat which is used in heating the water
in a liquid state to a temperature t°. On deducting this amount of heat, we obtain the
latent of evaporation of water as 60t>'5 at 0°, 571 at 50°, 534 at 100°, 494 at 150°, and only
453 at 200°, which shows that the conversion of water at different temperatures into
vapour at a constant temperature requires very different quantities of heat. This is
chiefly dependent on the difference of the cohesion of water at different temperatures ;
the cohesion is greater at low than at high temperatures, and therefore at low tem-
peratures a greater quantity of heat is required to overcome the cohesion. On comparing
these quantities of heat, it will be observed that they decrease rather uniformly,
namely their difference between 0° and 100° is 72, and between 100° and 200 3 is 81 units
of heat. From this we may conclude that -this variation will be approximately the same
for high temperatures also, and therefore that no heat would be required for the con-
version of water into vapour at a temperature of about 400° — 600D. At this temperature,
water passes into vapour whatever be the pressure (see chap. II. The absolute boiling
point of water, according to Dewar, is 370°, the critical pressure 196 atmospheres). It
must here be remarked that water, in presenting a greater cohesion, requires a larger
quantity of heat for its conversion into vapour than other liquids. Thus alcohol consumes
208, ether 90, turpentine 70, units of heat in their conversion into vapour.
The whole amount of heat which is consumed in the conversion of water into vapour
is not used in surmounting the cohesion — that is, in internal work accomplished in the
54
PRINCIPLES OF CHEMISTRY
purposes. The chemical reactions which water undergoes, and by
means of which it is formed, are so numerous, and so closely allied to
liquid. A part of this heat is employed in moving the aquj.ms particles; in fact, aqueous
vapour at 100° occupies a volume 1,650 times greater than that of water (at the ordinary
pressure), consequently a portion of the heat or work is employed in lifting the aqueous
particles, in overcoming pressure, or in external work, which may be usefully employed
and which is so employed in steam engines. In order to determine this work we will
first separately consider all the factors necessary for this calculation, and we will then
make a deduction from the comparison of these factors.
The maximum pressure or tension of aqueous vapour at different temperatures
has been determined with great exactitude by many observers. The observations of
Regnault in this respect, as on those preceding, deserve special attention from their
comprehensiveness and accuracy. The pressure or tension of aqueou* vapour at various
temperatures is given in the adjoining table, and is expressed in millimetres of the
barometric column having a temperature of 0°.
Tr;iiiu'r;tum- Tension
Temperature Trusimi
-20°
0-9
70° 233-3
— 10°
2-1
90° .VJ.V4
0°
4-6
100°
760-0
+ 10°
9-1 105°
90d'4
15°
12'7
110°
1075-4
20°
17-4
115°
1269'4
25°
23-5
120°
L491-8
30°
31-5
150° :j5Hl-o
50°
92-0
200° 110H9-0
The table shows the boiling points of water at different pressures. Thus on the
summit of Mont Blanc, where the average pressure is about 424 mm., water boils at
84*4°. In a rarefied atmosphere water boils at even the ordinary temperature, but in
evaporating it absorbs heat from the neighbouring parts, and therefore it becomes cold
and may even freeze if the pressure does not exceed 4'(5 mm., and especially if the vapour
be rapidly absorbed as it is formed. Oil of vitriol, which absorbs the aqueous vapour, is
Uried for this purpose. Thus ice may be obtained artificial!}' at the ordinary temperature
with the aid of an air-pump. This table of the tension of aqueous vapour also shows the
temperature of water contained in a closed boiler if the pressure of the steam formed l>e
known. Thus at a pressure of five atmospheres (a pressure of five times the ordinary
atmospheric pressure — i.e., 5x760 = 3,800 mm.) the temperature of the water would lie
152 '. The table also shows the pressure produced on a given surface by steam on issuing
from a boiler. Thus steam having a temperature of 152° exerts a pressure of 517 kilos, on a
piston whose surface equals 100 sq. c.m., for the pressure of one atmosphere on one
sq. c.m. equals 1,033 kilos., and steam at 152° has a pressure of five atmospheres. A>
a column of mercury 1 mm. high exerts a pressure of 1'35959 grams on a surface of
1 sq. c.m., therefore the pressure of aqueous vapour at 0° corresponds with a pressure of
6'25 grams per square centimetre. The pressures for all temperatures may be calculated
in a similar way, and it will be found that at 100° it is equal to ].o:;:;--2,s grams. This
means that if a cylinder be taken whose sectional area equals 1 sq. c.m.. and if water be
poured into it and it be closed by a piston weighing 1,0:!:! grams, thfii on heating it in a
vacuum to 100° no steam will be formed, because the steam cannot overcome the pressure
of the piston ; and if at 100° 534 units of heat be transmitted to each unit of weight of
water, {hen the whole of the water will be converted into vapour having the same
temperature ; and so also for every other temperature. The question now arises, To
what height does the piston rise under these circumstances ; that is, in other words, What
is the volume occupied by the steam under a known pressure ? For this we must know
ON WATKK AND ITS COMI'orNDS 55
the reactions of many other substances, that it is impossible to describe
tin- majority of them at this early stage of chemical exposition. After-
wards \vc shall become acquainted with many of them, but at present
w.' shall only cite certain compounds formed by water. In order to
see clearly the nature of the various kinds of compounds formed by
tin- weight df a cubic centimetre of steam at various temperatures. It has been shown by
experiment that the density of steam, which does not saturate a space, varies very
inconsiderably at all possible pressures, and is nine times the density of hydrogen under
similar conditions. Steam which saturates a space varies in density at different tem-
peratures, but this difference is very small, and its average density with reference to air is
OT>4. We will employ this figure in. our calculation, and will calculate what volume the
steam occupies at 100°. One cubic centimetre of air at 0° and 760 mm. weighs
0'00r2'.)3 gram, at 100 and under the same pressure it will weigh — or about
1°368
tr()UO'.»4(> gram, and consequently one cubic centimetre of steam whose density is 0'64
will weigh 0'000605 gram at 100°, and therefore one gram of aqueous vapour will
occupy a volume of about 1,653 c.c. Consequently, the piston in the cylinder of
1 sq. c.m. sectional area, and in which the water occupied a height of 1 c.m., will be
raised l,f>r>3 c.m. on the conversion of this water into steam. This piston, as has been
mentioned, weighs 1,033 grams, therefore the external icork of the steam — that is, that
work which the water does in its conversion into steam at 100° — is equal to lifting a piston
weighing 1,033 grains to a height of 1,653 c.m., or 17'07 kilogram-metres of work — i.e., is
capable of lifting 17 kilograms 1 metre, or 1 kilogram 17 metres. One gram of water
requires for its conversion into steam 534 gram units of heat or 0'534 kilogram units of
heat i.r., the quantity of heat absorbed in the evaporation of one gram of water is equal
to the quantity of heat which is capable of heating 1 kilogram of water 0'534°. Each
unit of heat, as has been shown by accurate experiment, is capable of doing 424 kilogram-
metres of work. Therefore, in evaporating, one gram of water expends 424xO'534 =
(almost) '226 kilogram-metres of work. The external work was found to be only
17 kilogram-metres, therefore 209 kilogram-metres are expended in overcoming the
internal cohesion of the aqueous particles, and consequently about 92 p.c. of the heat or
work consumed goes in overcoming the internal cohesion. The following figures are
thus calculated approximately : —
Total work of External work of T ,
•JVmpeniture evaporation in vapour in , "J " "
Kiln-ram -metres Kiln -ram-metres woikol \apom
0° 255 13 242
50° 242 15 227
100° 226 17 209
150° 209 ly 190
200° l'.)-2 20 172
Thus it will be remarked from this table that the work necessary for overcoming the
internal cohesion of water in its passage into vapour decreases with the rise in tempera-
ture; this is in connection with the decrease of cohesion with a rise in tempera-
ture, and, in fact, the variations which take place in this case are very similar to those
which are observed in the heights to which water rises in capillary tubes at different
t •lup.-ratures. It is evident, therefore, that the amount of external — or, as it is termed,
useful— work which water can supply by its evaporation is very small compared with the
am unit which it expends in its conversion into vapour.
IP. considering certain physico-meclianical properties of water, I had in view not only
their importance for theory and practice, but also their purely chemical significance, for
it is evident from the above considerations that in even a physical change of state the
greatest part of the work accomplished goes in overcoming cohesion, and that chemical
cohesion, or affinity, is an enormous internal energy.
56 I'KINCIPLKS OF CHEMISTRY
water we will begin with the most feeble, which are determined by
purely mechanical superficial properties of the reacting substances.12
Water is mechanically attracted by many substances ; it adheres to
their surfaces just as dust adheres to objects, and one polished glass
adheres to another. Such attraction is termed ' moistening,' ' soaking,' or
* absorption of water.' Thus water moistens clean glass and adheres to
its surface, is absorbed by the soil, sand, and clay, and does not flow
away from them but lodges itself between their particles. Similarly,
water soaks into a sponge, cloth, hair, or paper, etc., but fat and greasy
substances in general are not moistened. Attraction of this kind does
not alter the physical or chemical properties of water. For instance,
under these circumstances water, as is known from everyday experi-
ence, may be expelled from objects by drying. Water which is in any
way held mechanically may be dislodged by mechanical means, by fric-
tion, pressure, centrifugal force, <fcc. Thus water is squeezed from wet
cloth by pressure or centrifugal machines. But objects which in prac-
tice are called dry (because they do not wet people's hands) often still
contain moisture, as may be proved by heating the object in a glass
tube closed at one end. By placing a piece of paper, dry earth, or any
similar object (especially porous substances) in such a glass tube, and
heating that part of the tube where the object is situated, it will be
remarked that water condenses on the cooler portions of the tube. The
presence of such absorbed, or, as it is termed, ' hygroscopic,'' water is
generally best recognised in non- volatile substances by drying at 100°,
12 When it is necessary to heat a considerable mass of liquid in different vessels, it
would be very uneconomical to make use of metallic vessels and to construct a separate
fire grate under each one ; such cases are continually met with in practice. A considerable
mass of water, for instance, may have to be heated for making solutions, or it may be
required to expel volatile liquids from different vessels at intermittent periods ; as, for
instance, alcohol from partially fermented liquors, &c. In such cases one boiler or
vessel containing water is made use of. Steam from this boiler is introduced into the
liquid, or, in general, into the vessel which it is required to heat. The steam, in con-
densing and passing into a liquid state, parts with its latent heat, and as this is very
considerable a small quantity of steam will produce a considerable heating effect. If it
be required, for instance, to heat 1,000 kilos, of water from 20° to 50°, which requires
approximately 30,000 units of heat, steam heated to 100° is passed into the water from
a boiler. Each kilogram of water at 50° contains about 50 units of heat, and each kilo-
gram of steam at 100° contains 637 units of heat ; therefore, each kilogram of steam in
cooling to 50° gives up 587 units of heat, and consequently 52 kilos of steam are capable
of accomplishing the required heating of 1,000 kilos, of water from 20° to 50°. Water is
very often applied for heating in chemical practice. For this purpose metallic vessels
or pans, called ' water-baths,' are made use of. They are closed by a cover formed of
concentric rings lying on each other. The objects — such as beakers, evaporating basins,
retorts, &c. — containing liquids are placed on these rings, and the water in the bath is
heated. The steam given off heats the bottom of the vessels to be heated, and thus
accomplishes the evaporation or distillation or other required process. A water-bath
may also be used for heating a vessel directly immersed in the water.
()N WATKi; AND ITS CoMrol'NDS
57
or under the receiver of an air-pump and over substances which attract
water chemically. By weighing a substance before and after drying, it
is easy to determine the amount of hygroscopic water from the loss in
weight.13 Only in this case the amount of water must be judged with
13 In order t«« dry any substance at about 100°— that is, at the boiling point of water
(hygroscopic- water passes off at this temperature) — an apparatus called a ' drying-oven '
is employed. It consists of a double copper box ; water is poured into the space
between the internal and external boxes, and the oven is then heated over a stove or by
any other means, or else steam from a boiler is passed between the walls of the two
boxes. When the water boils, the temperature inside the inner box will be approximately
100° C. The substance to be dried is placed inside the oven, and the door is closed.
Several holes are cut in the door to allow the free passage of air, which carries off the
aqueous vapour by the chimney on the top of the oven. Often, however, desiccation is
carried on in copper ovens heated directly over a lamp fig. 13). In this case any desired
FIG. 13. — Drying oven, composed of brazc-d copper. It is heated by a lamp. The object to be dried
is placed on the gauze inside the oven. The thermometer indicates the temperature.
temperature may be obtained, which is determined by a thermometer fixed in a special
orifice. There are substances which only part with their water at a much higher
temperature than 100°, and then such air baths are very useful. In order to directly
determine the amount of water in a substance which does not part with anything except
water at a red heat, the substance is placed in a bulb tube. By first weighing the tube
empty and then with the substance to be dried in it, the weight of the substance taken may
be found. The tube is then connected on one side with a gas-holder full of air, which, on
opening a stop-cock, passes first through a flask containing sulphuric acid, and then into
a vessel containing lumps of pumice stone moistened with sulphuric acid. In passing
through these vessels the air is thoroughly dried, having given up all its moisture to the
sulphuric m-id. Thus dry air will pass into the bulb tube, and as hygroscopic water is
entirely given up from a substance in dry air at even the ordinary temperature, and still
58 PRINCIPLES OF CHEMISTRY
care, because the loss in weight may sometimes proceed from the de-
composition of the substance itself, with disengagement of gases or
vapour. In making exact weighings the hygroscopic capacity of sub-
stances— that is, their capacity to absorb moisture — must be continually
kept in view, as otherwise the weight will be untrue from the presence
of moisture. The quantity of moisture absorbed depends on the degree
of moisture of the atmosphere (that is, on the tension of the aqueous
vapour in it) in which a substance is situated. In an entirely dry
atmosphere, or in a vacuum, the hygroscopic water is expelled, being
converted into vapour ; therefore, if we have the means of drying yases
(or a vacuum) — that is, of removing the aqueous vapour from them —
objects impregnated with water may be entirely dried by placing them
in such a desiccated atmosphere. The process is aided by heat, as it
increases the tension of the aqueous vapour. Phosphoric anhydride (a
white powder), liquid sulphuric acid, solid and porous calcium chloride,
or the white powder of ignited copper sulphate are most generally
employed in drying gases. They absorb the moisture contained in air
and all gases to a considerable, but not unlimited, extent. Phosphoric
anhydride and calcium chloride deliquesce, become damp, sulphuric acid
changes from an oily thick liquid into a more mobile liquid, and ignited
copper sulphate becomes blue ; after which changes these substances
partly lose their capacity of holding water, and can, if it be in excess,
even give up their water to the atmosphere. We may remark that the
order in which these substances are placed above corresponds with the
order in which they stand in respect to their capacity for absorbing
moisture. Air dried by calcium chloride still contains a certain amount
of moisture, which it can give up to sulphuric acid. The most com-
plete desiccation takes place with phosphoric anhydride. Water is also
removed from many substances by placing them in a basin over a vessel
containing a substance absorbing water under a glass bell.14 The
bell, like the receiver of an air pump, should be hermetically closed.
more rapidly on heating, the moisture given up by the substance in the tube will be
carrietl off by the air passing through it. This damp air then passes through a U-shaped
tube full of pieces of pumice stone moistened with sulphuric acid, which absorbs all the
moisture given off from the substance in the bulb tube. Thus all the water expelled
from the substance will collect in the U tube, and so, if this be weighed before and after,
the difference will show the quantity of water expelled from the substance. If only water
(and not any gases) come over, the increase of the weight of the U tube will be equal to
the decrease in the weight of the bulb tube.
14 Instead of under a, glass bell, drying over sulphuric acid is often carried on in a
desiccator composed of a wide-mouthed low glass vessel, closed by a well-fitting ground-
glass stopper. Sulphuric acid is poured over the bottom of the desiccator, and the
substance to be dried is placed on a glass stand above the acid. A lateral glass tube with
a stop-cock is often fused into the desiccator in order to connect it with an air pump, and
so allow drying under a diminished pressure, when the moisture evaporates more rapidly.
(>.\ \VATKK AM) ITS m.MI>nrNI>S 59
In this case desiccation takes place ; because sulphuric acid, for instance,
iirst dries the air in the bell by absorbing its moisture, the substance
to he dried then parts with its moisture to the dry air, from which it is
again absorbed by the sulphuric acid, Arc. Desiccation proceeds still
better under the receiver of an air pump, for then the aqueous vapour
is formed more quickly than in a bell full of air.
From what has been said above, it is evident that the transference
of moisture to gases and the absorption of hygroscopic moisture present
un-at resemblance to, but still are not, chemical combinations with
water. Water, when combined as hygroscopic water, does not lose
its properties and does not form new substances.15
The attraction of water for substances which dissolve in it is of a
different character. In the solution of substances in water there pro-
ceeds a peculiar kind of indefinite combination ; there is formed a new
homogeneous substance from the two substances taken. But here also
the bond connecting the substances is very unstable. Water contain-
ing different substances in solution boils at a temperature near to its
usual boiling point, and acquires properties which are closely allied to
the properties of water itself and of the substances dissolved in it.
Thus, from the solution of substances which are lighter than water
itself, there are obtained solutions of a less density than water — as, for
example, in the solution of alcohol in water ; whilst a heavier sub-
stance in dissolving in water gives it a higher specific gravity. Thus
salt water is heavier than fresh.16
We will consider aqueous solutions somewhat fully, because, among
other reasons, solutions are constantly being formed on the earth and
in the waters of the earth, in plants and in animals, in chemical prac-
tice and in the arts, and these solutions play an important part in
the chemical transformations which are everywhere taking place, not
only because water is everywhere met with, but chiefly because a sub-
stance in solution presents the most propitious conditions for the process
of chemical changes, which require a mobility of parts and an intimate
1 ' Chapuy, however, determined that in wetting 1 gram of charcoal with water 7 units
of lieat are evolved, and on pouring carbon bisulphide over 1 gram of charcoal as much
as -Jl units of heat are evolved. Alumina (1 gram), wlien moistened with water, evolves
lories. This indicates that even in respect to evolution of heat moistening already
presents a transition towards exothermal combinations (those evolving heat in their
formation), like solutions.
"'• Strong acetic acid (CoH4O.j, whose specific gravity at 15° is T055, does not become
lighter on the addition of water (a lighter substance, sp. gr. = 0'99!)), but heavier, so that
a solution of so parts of acetic acid and 20 parts of water has a specific gravity of 1'074,
and even a solution of equal parts of acetic acid and water (50 p.c.) has a sp. gr. of T065,
which is still greater than that of acetic acid itself. This shows the high degree of con-
traction which takes place on solution. In fact, solutions — and, in general, liquids — on
mixing with water, decrease in volume.
60
PRINCIPLES OF CHEMISTRY
contact. In dissolving, a solid substance acquires a mobility of parts,
and a gas loses its elasticity, and therefore reactions often take place
in solutions which do not proceed in the undissolved substances. Fur-
ther, a substance, distributed in water, evidently breaks up (or ' disin-
tegrates ') — that is, becomes more like a gas and acquires a greater
mobility of parts. All these considerations require that in describing
Flo. 14. — Method of transferring a gas into a cylinder filled with mercurv and wlm.-e open end is im-
mersed under the mercury in a bath having two glass sides. The apparatus containing the gas is
represented on the right. Its upper extremity is furnished with a tube extending under the
cylinder. The lower part of the vessel communicates with a vertical tube. If mercury be poured
into this tube, the pressure of the gas in the apparatus is increased, and it passes tlm>ui:li the gag-
conducting tube into the cylinder, where it displaces the mercury, and can be measured or subjected
to the action of absorbing agents, such as water.
the properties of substances, particular attention should be paid to their
relation to water as a solvent.
Everybody knows that water dissolves many substances. Salt,
sugar, alcohol, and a number of other substances, by dissolving in water
form with it homogeneous liquids. To clearly show the solubility
of gases in water a gas should be taken which has a high co-efficient
of solubility — for instance, ammonia. This is introduced into a bell
(or cylinder, as in fig. 14), which is previously filled with mercury
and stands in a mercury bath. If water be then introduced into the
cylinder, the mercury will rise, owing to the water dissolving the
ammonia gas. If the column of mercury be less than the barometric
ON WATKK' AND ITS COMPOUNDS 61
column, and if there be sufficient water to dissolve the gas, all the
ammonia \vi\\ be absorbed by the water. The water is introduced into
tin- cylinder by a glass pipette, with a bent end. Its bent end is put
into water, and the air is sucked out from the upper end. When full
of water, its upper end is closed with the finger, and the bent end placed
in the mercury bath under the orifice of the cylinder. The water will
then be forced from the pipette by the atmospheric pressure, and will
i-i.se to the surface of the mercury in the cylinder owing to its lightness.
The solubility of a gas like ammonia may be demonstrated by taking a
flask full of the gas, and closed by a cork with a tube passing through
it. On placing the tube under water, the water will rise into the flask
(this may be accelerated by heating the flask), and begin to play like a
fountain inside it. Both the rising of the mercury and the fountain
clearly show the considerable affinity of water for ammonia gas, and the
force acting in this dissolution is rendered evident. For both the homo-
geneous intermixture of gases (diffusion) and the process of solution a
certain period of time is required, which depends, not only on the sur-
face of the participating substances, but also on their nature. This is
seen from experiment. Prepared solutions of different substances
heavier than water, such as salt or sugar, are poured into tall jars.
Pure water is then most carefully poured into these jars (through a
funnel) on to the top of the solutions, so as not to disturb the lower
stratum, and the jars are then left undisturbed. The line of demarca-
tion between the solution and the pure water will be visible, owing to
their different co-efficients of refraction. Notwithstanding that the
solutions taken are heavier than water, after some time complete inter-
mixture will ensue. Gay-Lussac convinced himself of this fact by
this particular experiment, which he conducted in the cellars under the
Paris Astronomical Observatory. These cellars are well known as the
locality where numerous interesting researches have been conducted,
because, owing to their depth under ground, they have a uniform tem-
perature during the wrhole year ; the temperature does not change
during the day, and this was indispensable for the experiments on the
diffusion of solutions, in order that no doubt in their results should
arise from a daily change of temperature (the experiment lasted several
months), which would set up currents in the liquids and intermix their
strata. Notwithstanding the uniformity of the temperature, the sub-
stance in solution in time ascended into the water and distributed itself
uniformly through it, proving that there exists between water and a
substance dissolved in it a particular kind of attraction or striving for
mutual interpenetration in opposition to the force of gravity. Further,
this effort, or rate of diffusion, is different for salt or sugar or for
62 PRINCIPLES OF CHEMISTRY
various other substances. Consequently, in solution there acts a
peculiar force, as in actual chemical combinations, and solution is de-
termined by a peculiar kind of movement (by the chemical energy of a
substance) which is proper to the substance dissolved and to the sol-
vent.
Graham made a series of experiments similar to those above
described, and he showed that the rate of diffusion*1 in water is very
variable — that is, a uniform distribution (under perfect rest, and with
such an arrangement of the strata of the solutions that uniformity
takes place in opposition to gravity) of a substance in the water dis-
solving it is attained in different periods of time with different solutions.
Graham compared diffusive capacity with volatility. There are sub-
stances which diffuse easily, and there are others which diffuse with
difficulty, just as there are more or less volatile substances. Seven
hundred cubic centimetres of water was poured into a jar, and by means
of a syphon (or a pipette) 100 cub. centimetres of a solution containing 10
grams of a substance was cautiously poured in so as to occupy the lower
portion of the jar. After the lapse of several days, successive layers of
50 cubic centimetres were taken from the top downwards, and the quan-
tity of substance dissolved in the different layers determined. Thus,
common table salt, after fourteen days, gave the following amounts (in
milligrams) in the respective layers, beginning from the top : 104. 120,
126, 198, 267, 340, 429, 535, 654, 766, 881, 991, 1,090, 1,187, and 2,266
in the remainder ; whilst albumin in the same time gave, in the first
seven layers, a very small amount, and beginning from the eighth layer,
10, 15, 47, 113, 343, 855, 1,892, and in. the remainder 6,725 milli-
grams. Thus, the diffusive power of a solution depends on time and
on the nature of the substance dissolved, which fact may serve, not only
for the explanation of the process of solution, but also in distinguishing
one substance from another. Graham showed that substances which
rapidly diffuse through liquids are able to rapidly pass through mem-
branes and crystallise, whilst substances which diffuse slowly and do not
crystallise are colloids, that is, resemble glue, and penetrate through
17 The researches of Graham, Fick, Nernst, and others showed that the quantity of a
dissolved substance which is transmitted (rises) from one stratum of liquid to another in
a vertical cylindrical vessel is not only proportional to the time and to the sectional area
of the cylinder, but also to the amount and nature of the substance dissolved in a stratum
of liquid, so that each substance has its corresponding co-efficient of diffusion. The cause
of the diffusion of solutions must be considered as essentially the same as the cause of
the diffusion of gases — that is, as dependent on movements which are proper to their
molecules ; but here most probably those purely chemical, although feebly-developed,
forces, which incline the substances dissolved to the formation of definite compounds,
also play their part.
ON WATKR AM» ITS COMPOUNDS M
;i nnMiibrane slowly, and form jellies ; that is, occur in insoluble
forms.18
18 The rate of diffusion — like the rateof transmission — through membranes, or tlitih/*/*
(which plays an important part in the vital processes of organisms and also in technical
work). present*, according to the researches of Graham, a sharply-defined change in
passing from such crystallisable substances as the majority of salts and acids to sub-
stances which are capable of giving jellies (gum, gelatin, il'c.). The former diffuse into
solutions and pass through membranes much more rapidly than the latter, and Graham
therefore distinguishes between cri/xtdlloid.i, which diffuse rapidly, and colloids, which
diffuse slowly. On breaking solid colloids into pieces, a total absence of cleavage is
remarked. The fracture of such substances is like that of glue or glass. It is termed a
• conchoidal ' fracture. Almost all the substances of which animal and vegetable bodies
consist are colloids, and this is, at all events, partly the reason why animals and plants
have such varied forms, which have no resemblance to the crystalline forms of the
majority of mineral substances. The colloid solid substances in organisms — that is, in
animals and plants — are usually soaked with water, and take most peculiar forms, of net-
works, of grannies, of hairs, of mucous, shapeless masses, Arc., which are quite different
from the forms taken by crystalline substances. When colloids separate out from solu-
tions, or from a molten state, they present a form which is similar to that of the liquid
from which they were formed. Glass maybe taken as the best example of this. Colloids
are distinguishable from crystalloids, not only by the absence of crystalline form, but by
many other properties which admit of clearly distinguishing both these classes of solids,
as was shown by the above-mentioned English scientific man, Graham. Nearly all
colloids are capable of passing, under certain circumstances, from a soluble into an
insoluble state. The best example is shown by white of eggs (albumin) in the raw and
soluble form, and in the hard-boiled
and insoluble form. The majority
of colloids, on passing into an in-
soluble form in the presence of
water, give substances having a
gelatinous appearance, which is
familiar to every one in starch,
solidified glue, jelly, itc. Thus
gelatin, or common carpenter's ^^^^f
glue, when soaked in water, swells
up into an insoluble jelly. If this
jelly be heated, it melts, and is then
soluble in water, but on cooling it
again forms a jelly which is in-
soluble in water. One of the pro-
perties which distinguish, colloids Fl(i- 15.- Dialyser. Apparatus for tie separation of sub-
, „ . , . ,, , ,, , stances winch jwss through a membrane from those
from crystalloids is that the former which do nor. Description in text.
pass very slowly through a mem-
brane, whilst the latter penetrate very rapidly. This maybe shown by taking a cylinder,
open at both ends, and by covering its lower end with a bladder or with vegetable parch-
ment (unsized paper immersed for two or three minutes in a mixture of sulphuric acid and
half its volume of water, and then washed), or any other membranous substance (all such
substances are themselves colloids in an insoluble form). The membrane must be firmly
tied to the cylinder, so as not to leave any opening. Such an apparatus is called a
fliuli/ser (fig. 15), and the process of separation of crystalloids from colloids by means of
such a membrane is termed dialysis. An aqueous solution of a crystalloid or colloid,
or a mixture of both, is poured into the dialyser, which is then placed in a vessel con-
taining water, so that the bottom of the membrane is covered with water. Then, after a
certain period of time, the crystalloid passes through the membrane, whilst the colloid,
if it does pass through at all, does so at an incomparably slower rate. The crystalloid
64 I'KIXCII'LES OF CMIK3IJSTKY
If it be desired to increase the rate of solution, recourse must
be had to stirring, shaking, or some such mechanical movement,
obliging the solution formed round the given substance to rise up-
wards if the solution be heavier than water. But if once a uniform
solution is formed, it will remain uniform if the temperature be
uniform, no matter how heavy the dissolved substance is, or how long
the solution be left at rest, which fact again shows the presence of a
force holding together the particles of the body dissolved and of the
solvent.19
naturally passes through into the water until the solution attains the same strength on
both sides of the membrane. By replacing the outside water with fresh water, a fresh
quantity of the crystalloid may be separated from the dialyser. While a crystalloid is
passing through the membrane, a colloid remains almost entirely in the dialyser, and
therefore a mixed solution of these two kinds of substances may be separated from each
other by a dialyser. The study of the properties of colloids, and of the phenomena of
their passage through membranes, should elucidate much respecting the phenomena
which are accomplished in organisms.
19 The formation of solutions may be considered in two aspects, from a physical and from
a chemical point of view, and it is more evident in solutions than in any other department
of chemistry that these provinces of natural science are allied together in a most intimate
manner. On one hand solutions form a particular aspect of a physico-mechanical inter-
penetration of homogeneous substances, and a juxtaposition of the molecules of the sub-
stance dissolved and of the solvent, similar to the juxtaposition which is exhibited in
homogeneous substances. From this point of view this diffusion of solutions is exactly
similar to the diffusion of gases, with only this difference, that the nature and store of
energy is different in gases from what it is in liquids, and th»,t in liquids there is consider-
able friction whilst in gases there is comparatively little. The penetration of a dissolved
substance into water is likened to evaporation, and solution to the formation of vapour.
This resemblance was clearly expressed even by Graham. In recent years the Dutch
chemist, Van't Hoff , has developed this view of solutions in great detail, having shown (in
a memoir in the Transactions of the Swedish Academy of Science, Part 21, No. 17,
' Lois de 1'equilibre chimique dans Petat dilue, gazeux au dissous,' 1886), that for dilute
solutions the osmotic pressure follows the same laws (of Boyle, Mariotte, Gay-Lussac.
and Avogadro-Gerhardt) as for gases. The osmotic pressure of a substance dissolved in
water is determined by means of membranes which allow the passage of water, but not
of a substance dissolved in it, through them. This property is found in animal proto-
plasmic membranes and in porous substances covered with an amorphous precipitate
such as is obtained by the action of copper sulphate on potassium ferrocyanide (Pffeifer
Traube). If, for instance, a one p.c. solution of sugar be placed in such a vessel,
which is then closed and placed in water, then the water passes through the walls
of the vessel and increases the pressure by 50 mm. of the barometric column. If the
pressure be artificially increased inside the vessel, then the water will be expelled
through the walls. The osmotic pressure of dilute sohitions determined in this manner
(from observations made by Pffeifer and De Vries) was shown to follow the same laws
as those of the pressure of gases ; for instance, by doubling or increasing the quantity of
a salt (in a given volume) n times, the pressure is doubled or increases n times. One of
the extreme consequences of the resemblance of osmotic pressure to gaseous pressure
is that the concentration of a uniform solution varies in parts which are heated or cooled.
Soret (1881) indeed observed that a solution of copper sulphate containing 17 parts of
the salt at 20° only contained 14 parts after heating the upper portion of the tube to
80° for a long period of time. This aspect of solution, which is now being very carefully
and fully worked out, may be called the physical side. Its other aspect is purely
chemical, for solution does not take place between any two substances, but requires a
<>N YYATKK AND ITS COMPOUNDS 65
In the consideration of the process of solution, besides the con-
ception of diffusion, another fundamental conception is necessary,
namely, that of the saturation of solutions.
^pecial and particular attraction or affinity between them. A vapour or gas permeates
into any other vapour or gas, but a salt which dissolves in water may not be in the least
soluble in alcohol, and is quite insoluble in mercury. In considering solution as a mani-
festation of chemical forces (and of chemical energy), it must be acknowledged that they
an- here developed to so feeble an extent that the definite compounds (that is, those
Formed according to the law of multiple proportions) which are formed between water
and a soluble substance dissociate at even the ordinary temperature, forming a homo-
geneous system— that is, one where both the compound and the products into which it
decomposes (water and the aqueous compound) occur in a liquid state. The chief diffi-
culty in the comprehension of solutions depends on the fact that the mechanical theory
of the structure of liquids has not yet been so fully developed as the theory of gases, and
solutions are liquids. The conception of solutions as liquid dissociated definite chemical
compounds is based on the following considerations : (1) that there exist certain undoubt-
edly definite chemical crystalline compounds (such as H2SO4, H2O ; or NaCl, 10H2O ; or
CaClo, 6HoO ; ivrc.) which melt on a certain rise of temperature, and then form real solu-
tions ; (2) that metallic alloys in a molten condition are real solutions, but on cooling they
often give entirely distinct and definite crystalline compounds, which are recognised by
the properties of alloys; (3) that between the solvent and the substance dissolved there
are formed, in a number of cases, many undoubtedly definite compounds, such as com-
pounds with water of crystallisation ; (4) that the physical properties of solutions, and
especially their specific gravities (a property which is very accurately observable), vary
with a change in composition, and in such a manner as the formation of one or several
definite but dissociating compounds would require. Thus, for example, on adding
water to fuming sulphuric acid its density is observed to decrease until it attains the
definite composition H2SO4, or SO3 + H2O, when the specific gravity increases, although
on further diluting with water it again falls. Further (Mendeleeff, The Investigation of
Aqueous Solution* from their Specific Gravities, 1887), the increase in specific gravity
(ds), with the augmentation (dp) of the percentage amount of a substance dissolved,
varies in all well-known solutions with the percentage amount of the substance dissolved,
so that a rectilinear dependence is obtained (i.e., ( S = A + B») between the limits of
dp
definite compounds which must be acknowledged to exist in solutions ; this would be
expected to be the case from the dissociation hypotheois. So, for instance, from H2SO4
to H2SO4 + H2O (both these substances exist as definite compounds in a free state), the
fraction ( S = 0'0729-0'000749p (where p is the percentage amount of H2SO4). For
alcohol C2H6O, whose aqueous solutions have been more accurately investigated than all
others, three definite compounds must be acknowledged in its solutions, C2HCO -f 12H2O,
C2H(,O + 3H2O, and 3C2H6O + H2O.
The two aspects of solution above mentioned, and the hypotheses which have as yet
been applied to the examination of solutions, although they have partially different
starting points, yet will doubtless in time lead to a general theory of solutions, because
the same common laws govern both physical and chemical phenomena, inasmuch as the
properties and movements of molecules, which determine physical properties, are depend-
ent on the movements and properties of atoms, which determine chemical mutual actions.
For details of the questions dealing with the theories of solution recourse must now be
had to special memoirs and to works on theoretical (physical) chemistry; for this subject
forms one of special interest at the present epoch of the development of our science.
In working out chiefly the chemical side of solutions I consider it to be necessary to
reconcile the two aspects of the question; this seems to me. to be all the more possible,
as the physical side is limited t,o dilute solutions only, whilst the chemical side deals
mainly with strong solutions.
VOL. -I. P
66 PRINCIPLES OF CHEMISTRY
Just as damp air may be added to any quantity of dry air it be
desired, so also a solvent liquid may be taken in an indefinitely large
quantity and yet a uniform solution will be obtained. But more than
a definite quantity of aqueous vapour cannot be introduced into a
certain volume of air at a certain temperature. The excess above the
point of saturation will remain in the liquid form.-0 The relation
between water and substances dissolved in it is similar. More than a
definite quantity of a substance cannot, at a certain temperature, dis-
solve in a given quantity of water ; the excess does not unite with the
water. Just as air or a gas becomes saturated with vapour, so water
becomes saturated with a substance dissolved in it. If an excess of a
20 A juxtaposition of (chemically or physically) reacting substances taken in various
states — for instance, some solid, others liquid or gaseous — is termed ft heterogeneous system.
Up to now it is only systems of this kind which can be subjected to d- 'tailed examination
in the sense of the mechanical theory of heat. Solutions present liquid homogeneous
systems, which as yet are subjected to investigation with difficulty.
In the case of limited solution of liquids in liquids, the difference hi-firci'it tJn
and the substance dissolved is clearly seen. The former (that is. the solvent) may be
added in an unlimited quantity, and yet the solution obtained will always be uniform,
whilst of the substance dissolved there can only be taken a definite saturating propor-
tion. We will take water and common (sulphuric) ether. On shaking the ether with the
water it will be remarked that a portion of it dissolves in the water, forming a solution.
If the ether be taken in such a quantity that it saturates the water and a portion of it
remains undissolved, then this remaining portion will act as a solvent, and water will
diffuse through it and also form a saturated solution of water in the ether taken. Thus
two saturated solutions will be obtained. One solution will contain ether dissolved in
water, and the other solution will contain water dissolved in ether. These two solutions
will arrange themselves in two layers, according to their density; the ethereal solution
of water will be on the top, as the lightest, and the aqueous solution of ether at the
bottom, as the heaviest. If the upper ethereal solution be poured off from the aqueous
solution, any quantity of ether may be added to it; this shows that the dissolving sub-
stance is ether. If water be added to it, it is no longer dissolved in it : this shows that
water saturates the ether — here water is the substance dissolved. If we act in the same
manner with the lower layer, we shall find that water is the solvent and ether the sub-
stance dissolved. By taking different amounts of ether and water, the degree of
solubility of ether in water, and of water in ether, may be easily determined. Thus, for
example, in the above case it is found that water approximately dissolves ^ of its
volume of ether, and ether dissolves a very small quantity of water. Let. us imagine that the
liquid poured in dissolves a considerable amount of water, and thai water dissolves a
considerable amount of the liquid. For instance, let us imagine that the saturation of
100 parts of water require 80 parts of the liquid, and that 100 parts of the liquid would
require 125 parts of water for its saturation. What would then take place if the liquid
be poured iu water ? Two layers could not be formed, because the saturated solutions
would resemble each other, and therefore they would intermix in all proportions.
Indeed, in the saturated aqueous solution there, would be 0'8 parts of the liquid taken to
1 part of water, and in the solution of water in the liquid taken there would be on
saturation 1 part of water to 0'8 parts of the liquid. There would be no line of demarca-
tion between the layers of the liquids, or, in other words, they would intermix in all
proportions. This is, consequently, a case of a phenomena where two liquids present
considerable co-efficients of solubility in each other, but where it is impossible to say what
these co-efficients are, because it is impossible to obtain a saturated solution.
ON WAT Hi; AND ITS COMPOUNDS
67
substance !>«' added to water which is already saturated with it, it will
remain in its original state, ;:nd will not spread through the water. The
quantity of a substance
(either l>y volume with
gases, or by weight with
solids and liquids) which is
capable of saturating 100
parts of water is called the
co-pftifii-Ht <>f xoltihilitij or
the sot nullify. Tn 100 grams
of water at 15°, there can
be dissolved not more than
35 "86 grams of common
salt. Consequently, its
solubility at 15° is equal
to 35-S6.21 It is most
-1 The solubility, or co-efficient
of solubility, of a substance is de-
termined by various methods.
Either a solution is expressly pre-
pared with a clear excess of the
soluble substance and saturated
at a given temperature, and the
quantity of water and of the sub-
stance dissolved in it determined
by evaporation, desiccation, or
other means ; or else, as is done
with gases, known quantities of
wat«-r and of the soluble sub-
stance are taken, and the amount
remaining undissolved is deter-
mined.
The solubility of a gas in water
is determined by means of an ap-
paratus called an absorptio-
niffcr (fig. 16). It consists of an
iron stand/, on which an india-rub-
ber ring rests. A wide glass tube
is plar-ed on this ring, and is pres-
sed down on it by the ring // and
fhe screws ii. The tube is thus
firmly fixed on the stand. A cock
r, communicating with a funnel r,
passes into the lower part of the
stand. Mercury can be poured
into the wide tube through this
funnel, which is therefore made
of steel, as copper would be
affected by the mercury. The
upper ring h is furnished with a
Ktui-rti's alisorjitionieter. Apparatus
niiiiiiiL' tlic solubility of gases in liquids.
leter-
P 2
68 PRINCIPLES OF CHEMISTRY
important to turn attention to the existence of the solid imsohi1>J<'
substances of nature, because on them depends the shape of the
cover 2^, which can be firmly pressed down on to the wide tube, and hermetically closes it
by means of an india-rubber ring. The tube r r can be raised at will, and so by pouring mer-
cury into the funnel the height of the column of mercury, which produces pressure inside
the apparatus, can be increased. The pressure can also be diminished at will, by letting
mercury out through the cock r, A graduated tube e, containing the gas and liquid to be
experimented on, is placed inside the wide tube. This tube is graduated in millimetres
for determining the pressure, and it is calibrated for volumes, so that the number of
volumes occupied by the gas and liquid dissolving it can be easily calculated. This tube
can also be easily removed from the apparatus. To the right of the figure, the lower
portion of this tube when removed from the apparatus is shown. It will be observed
that its lower end is furnished with a male screw 6, fitting in a nut a. The lower
surface of the nut a is covered with india-rubber, so that on screwing up the tube its
lower end presses upon the india-rubber, and thus hermetically closes the whole tube, for
its upper end is fused up. The nut a is furnished with arms c c, and in the stand f
there are corresponding spaces, so that when the screwed-up internal tube is fixed into
stand/, the arms c c fix into these spaces cut in/. This enables the internal tube to In-
fixed on to the stand/. When the internal tube is fixed in the stand, the wide tube is put
into its right position, and mercury and water are poured into the space between the two
tubes, and communication is opened between the inside of the tube e and the mercury
between the interior and exterior tubes. This is done by either revolving the interior
tube e, or by a key turning the nut about the bottom part of/. The tube e is filled with
gas and water as follows : the tube is removed from the apparatus, filled with mercury,
and the gas to be experimented on is passed into it. The volume of the gas is measured,
the temperature and pressure determined, and the volume it would occupy at 0° and
760 mm. calculated. A known volume of water is then introduced into the tube. The
water must be previously boiled, so as to be quite freed from air in solution. The tube is
then closed by screwing it down on to the india-rubber on the nut. It is then fixed on to
the stand/, mercury and water are poured into the intervening space between it and the
exterior tube, which is then screwed up and closed by the cover j?, and the whole
apparatus is left at rest for some time, so that the tube e, and the gas in it, may attain the
same temperature as that of the surrounding water, which is marked by a thermometer
Jc tied to the tube e. The interior tube is then again closed by revolving it in the nut,
the cover^? again shut, and the whole apparatus is shaken in order that the gas in the
tube e may entirely saturate the water. After several shakings, the tube e is again
opened by revolving it in the nut, and the apparatus is left at rest for a certain time ; it is
then closed and again shaken, and so on until the volume of gas does not diminish after
a fresh shaking — that is, until saturation ensues. Observations are then made of the
temperature, the height of the mercury in the interior tube, and the level of the water in
it, and also of the level of the mercury and water in the exterior tube. All these data
are necessary in order to calculate the pressure under which the solution of the gas takes
place, and what volume of gas remains undissolved, and also the quantity of water which
serves as the solvent. By varying the temperature of the surrounding water, the amount
of gas dissolved at various temperatures may be determined. Bunsen, Carius, and
many others determined the solution of various gases in water, alcohol, and certain
other liquids, by means of this apparatus. If in a determination of this kind it is found
that n cubic centimetres of water at a pressure h dissolve in cubic centimetres of a
given gas, measured at 0° and 760 mm., when the temperature under which solution
took place was t° and pressure h mm., then it follows that at the temperature / flic,
co-efficient of solubility of the gas in 1 volume of the liquid will be equal to m x ' '
This formula is very clearly understood from the fact that the co-efficient of solubility
of gases is that quantity measured at 0° and 760 mm., which is absorbed at a pressure
ON WATER AND ITS COMPOUNDS 69
substance of the earth's surface, and of plants and animals. There
is so much water on the earth's surface, that were the surface of sub-
stances formed of soluble matters it would constantly change, and
however substantial their forms might be, mountains, river banks and
s«-u shores, plants and animals, or the habitations and coverings of men,
could not exist for any length of time.22
of 7<>0 mm. by one volume of a liquid. If n cubic centimetres of water absorb m cubic
•centimetres of a gas, then one cubic centimetre absorbs — . If — c.c. of a gas are ab-
n n
sorbecl under a pressure of It mm., then, according to the law of the variation of
solubility of a ^as with the pressure, there would be dissolved, under a pressure of
760 mm., a quantity varying in the same ratio to — • as 760 : h. In determining the
residual volume of gas its moisture (note 1) must be taken into consideration.
Below are given the number of grams of several substances saturating 100 grams of
water — that is, their co-efficients of solubility by weight at three different temperatures : —
At 0° At 20° At 100°
, Oxygen, O2
Carbonic anhydride, CO2
"^fe ^ _
I Ammonia, NH3 I 90-0 51-8 7'3
, Phenol, CfiH«O ! 4'9 5'2 oo
Liquids Ainyl ak-oliol, C-.H^O . . . . 4'4 2'9
^ Sulphuric acid, H,>SO4 .... oo OO OO
( Gypsum, CaSO4 , 2HoO . j ^ i ^
] Alum, AlKSoOg , 12H..O ...... | 8'3 15'4 857'5
Solids - Anhydrous sodium sulphate, NaoSO4
Common Salt, NaCl
Nitre, KN03
4-5 20
85-7 86-0
18-8 81-7
43
39'7
Sometimes a substance is so slightly soluble that it may be considered as insoluble.
Many such substances are met with both in solids and liquids, and such a gas as oxygen,
although it does dissolve, does so in so small a proportion by weight that it might be
considered as zero did not the solubility of even so little oxygen play an important part
in nature (as in the respiration of fishes) and were not an infinitesimal quantity of a gas
by weight so easily measured by volume. The sign QO, which stands on a line with sul-
phuric acid in the above table, indicates that it intermixes with water in all proportions.
There are many such cases among liquids, and everybody knows, for instance, that spirit
{absolute alcohol) can be mixed in any proportion with water. Common corn spirit
(vodky) is a mixture of about fifty parts by weight of pure spirit to 100 parts by weight
of water.
22 Just as the existence must be admitted of substances which are completely un-
decomposable (chemically) at the ordinary temperature — for there are substances which
are entirely non-volatile at such a temperature (as wood and gold), although capable of
decomposing (wood) or volatilising (gold) at a higher temperature— so also the existence
must be admitted of substances which are totally insoluble in water without some degree
of change in their state. Although mercury is partially volatile at the ordinary tem-
perature, there is no reason to think that it and other metals are soluble in water, alcohol,
or other similar liquids. However, mercury forms solutions, as it dissolves other metals.
On the other hand, there are many substances found in nature which are so very
slightly soluble in water, that in ordinary practice they may be considered as insoluble
<for example, barium sulphate). For the comprehension of that general plan according to
which a change of state of substances (combined or dissolved, solid, liquid, or gaseous)
70 I'KIXCIPLKS OF CHEMISTRY
Substances which are easily soluble in water bear a certain resrin
blance to it. Thus sugar and salt in many of their superficial features
remind one of ice. , Metals, which are not soluble in water, have 110
points in common with it, whilst on the other hand they dissolve each
other in a molten state, forming alloys, just as oily substances dissolve
each other ; for example, tallow is soluble in petroleum and in olive oil,
although they are all insoluble in water. From this it is evident that
the analogy of substances forming a solution plays an important part,
and as aqueous and all other solutions are liquids, there is good reason to
believe that in the process of solution solid and gaseous substances
change in a physical sense, passing into a liquid state. These con-
siderations elucidate many points of solution — as, for instance, the vari-
ation of the co-efficient of solubility with the temperature and the evo-
lution or absorption of heat in the formation of solutions.
The solubility — that is, the quantity of a substance necessary for
saturation — varies with the temperature, and, further, with an increase
in temperature the solubility of solid substances generally increases, and
that of gases decreases ; this might be expected, as solid substances by
heating, and gases by cooling, approach to a liquid or dissolved state.23
A graphic method is often employed to express the variation of solu-
bility with temperature. On the axes of abscissae or on a horizontal
line, temperatures are marked out and perpendiculars are raised corre-
sponding with each temperature, whose length is determined by the
solubility of the salt at that temperature — expressing, for instance, one
part by weight of a salt in 100 parts of water by one unit of length,
such as a millimetre. By joining the summits of the perpendiculars,
a curve is obtained which expresses the degree of solubility at different
temperatures. For solids, the curve is generally an ascending one — i.e.*
recedes from the horizontal line with the rise in temperature. These
curves clearly show by their inclination the degree of rapidity of increase
in solubility with the temperature. Having determined several points
takes place, it is very important to make a distinction at this boundary line (on approach-
ing zero of decomposition, volatility, or solubility) between an insignificant amount and
zero, but the present methods of research and the data at our disposal at the present
time do not yet touch such questions. It must be remarked, besides, that water in a
number of cases does not dissolve a substance as such, but acts on it chemically and forms
a soluble substance. Thus glass and many rocks, especially if taken as powder, are
chemically changed by water, but are not directly soluble in it.
23 Beilby (1883) experimented on paraffin, and found that one cubic decimetre of solid
paraffin at 21° weighed 874 grams, and when liquid, at its melting-point 88°, 788 grams, at
49°, 775 grams, and at 60°, 767 grams, from which the weight of a litre of liquefied paraffin
would be 795-4 grams at 21° if it could remain liquid at that temperature. By dissolving
solid paraffin in lubricating oil at 21° Beilby found that 795'6 grams occupy one cubic
decimetre, from which he concluded that the solution contained liquefied paraffin.
o.\ WATEB AND ITS COMPOUNDS 71
of ;i curve that i>, having made a determination of the solubility for
several temperature tin- solubility at intermediary temperatures may
be determined from the sinuosity and form of the curve so formed ; in
this way the empirical law of solubility may be followed.'2"1 The results of
research have shown that the solubility of certain salts — as, for example,
(in 11 moil table salt — varies comparatively little with the temperature ;
whilst for other substances the solubility increases by equal amounts for
equal increments of temperature. So, for example, for the saturation of
-' (lay-Lu-^ac \vas the first to have recourse to such a graphic method of expressing
solubility, and lie considered, in accordance with the general opinion, that by joining up
the summits of the ordinates in one harmonious curve it is possible to express the entire
change of solubility with the temperature. Now, there are many reasons for doubting
the accuracy of such an admission, for there undoubtedly are critical points in curves of
solubility (for example, of sodium sulphate, as shown further on), and it may be that
definite compounds of dissolved substances with water, in decomposing within known
limits of temperature, give critical points more often than would be imagined; it may
even be, indeed, that instead of a continuous curve, solubility should be expressed — if
not always, then not unfrequently — by straight or broken lines. According to Ditte, the
solubility of sodium nitrate, NaXO,-, is expressed by the following figures per 100 parts of
water : —
0° 1 10° 15° 21° 29° 36° 51° 68°
()C>-7 71-0 7<i'o 80-6 85'7 92".> 91)'4 13'6 12;V1
According to my opinion (iHHlj, these data should be expressed with exactitude by a
straight line. (\7',~> -f O'STf, which entirely agrees with the results of experiment. Accord-
ing to this the figure expressing the solubility of the salt at 0° exactly coincides with
the composition of a definite chemical compound — NaXO5,7H2O. The experiments
made by Ditte showed that all saturated solutions between 0° and — 15'7 have such a
composition, and that at the latter temperature the solution completely solidifies into one
homogeneous whole. Ditte shows, in the first place, that the solubility of sodium nitrate
is expressed by a broken straight line, and, in the second place, confirms the idea,
which I had already traced, that in solutions we have definite chemical compounds in a
state of dissociation. In recent times (IHHH) Etard discovered a similar phenomenon in
many of the sulphates. Brandes, in 1830, shows a diminution in solubility below 100°
for manganese sulphate. The percentage by weight (i.e., per 100 parts of the solution, and
not of wateri of saturation for ferrous sulphate, FeSO4, from — 2° to + 65° = 13'5 + 0'3784f —
that is, the solubility of the salt increases. The solubility remains constant from 65° to
98° (according to Brandes the solubility then increases ; this divergence of opinion
requires proof), and from 98° to 150° it falls as = 104'35- 0'6685£. Hence, at about
+ 156° the solubility should =0, and this has been confirmed by experiment. I observe,
on my side, that Etard's formula gives 38'1 p.c. of salt at (55° and 38"8p.c. at 92°, and this
maximum amount of salt in the solution very nearly corresponds with the composition
FeSO4,14H2O, which requires 37'6 p.c. Thus, in this case, as in that of sodium nitrate,
the formation of a definite solution may be presupposed. From what has been said, it is
evident that the data concerning solubility require a new method of investigation, which,
in the first place, should have in view the entire scale of solubility— from the formation
of completely solidified solutions (cryohydrates, which we shall speak of presently) to the
separation of salts from their solutions, if this is accomplished at a higher temperature
(for manganese and cadmium sulphates there is an entire separation, according to Etard),
or to the formation of a constant solubility (forpotassium sulphate the solubility, accord-
ing to Etard, remains constant from 163° to 220° and equals 24'9 p.c.) ; and, in the second
place, should endeavour to apply the conception of definite compounds existing in solu-
tions to constant and critical solutions, corresponding with a maximum of solubility or
of its limits. From these aspects solution should present a new and particular inter.
7k2 PRINCIPLES OF CHEMISTRY
100 parts of water by potassium chloride there is required at 0°, 29*2
parts, at 20°, 34*7, at 40°, 40'2, at 60°, 45-7 ; and so on, for every 10°
the solubility increases by 2 -75 parts by weight of the salt. Therefore
the solubility of potassium chloride in water may be expressed by a
direct equation : a=29*2 + 0*2752, where a represents the solubility at t".
For other salts, more complicated equations are required. For exam pie,
for nitre: a=13*3 + 0*5742 + 0*0171722 + O0000036*3, which shows
that when 2=0° a=13*3, when 2 = 10° a=20'8, and when 2 = 100°
a=246*0.
Curves of solubility give the means of judging with accuracy the
amount of a salt separated by the cooling to a known extent of a
solution saturated at a given temperature. For instance, if 200 parts
of a solution of potassium chloride in water saturated at a temperature
of 60° be taken, and it be asked how much of the salt will be separated
by cooling the solution to 0°, if its solubility at 60° = 45'7 and at
0°=29*2 ? The answer is obtained in the following manner : At 60° a
saturated solution contains 45*7 parts of potassium chloride per 100
parts by weight of water, consequently 145*7 parts by weight of the
solution contains 45*7 parts, or, by proportion, 200 parts by weight of
the solution contains 62*7 parts of the salt. The amount of salt
remaining in solution at 0° is calculated as follows : In 200 grams
taken there will be 137*3 grams of water ; consequently, this amount of
water is capable of holding only 40*1 grams of the salt, and therefore
in lowering the temperature from 60° to 0° there should separate from
the solution 62*7 — 40*1 = 22*6 grams of the dissolved salt.
The difference in the solubility of salts, <fcc., with a rise or fall of
temperature is often taken advantage of, especially in technical
work, for the separation of salts in intermixture from each other.
Thus a mixture of potassium and sodium chlorides (this mixture is met
with in nature at Stassfiirt) is separated from a saturated solution by
subjecting it alternately to boiling (evaporation) and cooling. The
sodium chloride separates out in proportion to the amount of water
expelled from the solution by boiling, and is removed, whilst the
potassium chloride separates out on cooling, as the solubility of this
salt rapidly decreases with a lowering in temperature. Nitre, sugar, and
many other soluble substances are purified (refined) in a similar
manner.
Although in the majority of cases the solubility of solids increases
with the temperature, yet just as there are substances whose volume
diminishes with a rise in temperature (for example, water from 0° to
4°), so there are not a few solid substances whose solubilities fall on
heating. Glauber's salt, or sodium sulphate, historically forms a particu-
ON WATER AND ITS COMPOUNDS 78
larly instructive example of the case in question. If this salt be taken
in an ignited state (deprived of its water of crystallisation), then its
solubility in 100 parts of water varies with the temperature in the
following manner : at 0°, 5 parts of the salt form a saturated solution ;
at 20°, 20 parts of the salt, at 33° more than 50 parts. As will be
seen, the solubility increases with the temperature, as is the case
with nearly all salts ; but starting from 33° it suddenly diminishes,
and at a temperature of 40°, there dissolves less than 50 parts of
the salt, at 60° only 45 parts of the salt, and at 100° about 43
parts of the salt in, 100 parts of water. This phenomenon may be
traced to the following facts : Firstly, that this salt forms various
compounds with water, as will be afterwards explained ; secondly,
that at 33° the compound Na2SO4 + 10H.20 formed from the solu-
tion at lower temperatures, melts ; and thirdly, that on evaporation
at a temperature above 33° there separates out an anhydrous salt,
Na2S04. It will be seen from this example how complicated such a
seemingly simple phenomenon as solution really is ; and all data con-
cerning solutions lead to the same conclusion. This complexity becomes
evident in investigating the heat of solution. If solution consisted of
a physical change only, then in the solution of gases there would be
evolved — and in the solution of solids, there would be absorbed — so
much heat as answers to the change of state ; but in reality a large
amount of heat is always evolved in solution, depending on the fact
that in the process of solution there is accomplished an act of chemical
combination, accompanied by an evolution of heat. Seventeen grams of
ammonia (this weight corresponds with its formula NH3), in passing
from a gaseous into a liquid state, evolve 4,400 units of heat (latent
heat) ; that is, the quantity of heat necessary to raise the temperature
of 4,400 grams of water 1°. The same quantity of ammonia, in dissolv-
ing in an excess of water, evolves twice as much heat — namely 8,800
units — showing that the combination with water is accompanied by the
evolution of 4,400 units of heat. Further, the chief part of this -heat
is separated in dissolving in small quantities of water, so that 17 grams
of ammonia, in dissolving in 18 grams of water (this weight corre-
sponds with its composition H2O), evolve 7,535 units of heat, and there-
fore the formation of the solution NH3 + H2O evolves 3,135 units of
heat beyond that due to the change of state. As in the solution of
gases, the heat of liquefaction (of physical change of state) and of chemi-
cal combination with water are both positive ( + ), therefore in the
solution of gases in water a heat effect is alwa}*s observed. This pheno-
menon is different in the solution of solid substances, because their
passage from a solid to a liquid state is accompanied by an absorption
74 PRINCIPLE.-
of heat (negative, — heat), whilst their chemical combination with water
is accompanied by an evolution of heat ( 4- heat) ; consequently, their
sum may either be a cooling effect, when the positive (chemical) portion
of heat is less than the negative (physical), or it may be, on the
contrary, a heating effect. This is actually the case. 124 grams of
sodium thiosulphate (employed in photography) Na,SsO3,5H20 in
melting (at 48°) absorbs 9,700 units of heat, but in dissolving in a large
quantity of water at the ordinary temperature it absorbs 5,700 units of
heat, which shows the evolution of heat (about + 4,000 units), not-
withstanding the cooling effect observed in the process of solution, in
the act of the chemical combination of the salt with water.-' But in
25 The latent heat of fusion is determined at the temperature of fusion, whilst solution
takes place at the ordinary temperature, and one must think that at this temperature
the latent heat would be different, just as the latent heat of evaporation varies with the
temperature (see note 11, p. 52). Besides which, in solution there occurs a disunion (dis-
integration) of the particles of both the solvent and the substance dissolved, which in its-
mechanical aspect resembles evaporation, and which therefore must consume much
heat. The heat emitted during the solution of a solid must be therefore considered
(Personne) as composed of three factors — (1) positive, the effect of combination; (2).
negative, the effect of transference into a liquid state ; and (3) negative, the effect of dis-
integration. In the solution of a liquid by a liquid the second factor is removed ; and
therefore if the heat evolved in combination is greater than that absorbed in disintegra-
tion a heating effect is observed, and in the reverse case a cooling effect ; and, indeed,
sulphuric acid, alcohol, and many liquids evolve heat in dissolving in each other. But the
solution of chloroform in carbon bisulphide (Bussy and Binget), or of phenol (or aniline)
in water (Alexeeff), produces cold. In the solution of a small quantity of water in acetic
acid (Abasheff), or hydrocyanic acid (Bussy and Binget), or amyl alcohol (Alexeeff), cold
is produced, whilst in the solution of these substances in an excess of water heat is
evolved.
The fullest information concerning the solution of liquids in liquids has been
gathered by W. T. Alexe'eff (1883-1885), still these data are far from being sufficient to
resolve the mass of problems respecting this subject. He showed that two liquids which
dissolve in each other, intermix together in all proportions at a certain temperature.
Thus the solubility of phenol, C6H6O, in water, and the converse, is limited up to
70°, whilst above this temperature they intermix in all proportions. This is seen
from the following figures, where p is the percentage amount of phenol and t the
temperature at which the solution becomes cloudy — that is, that at which it is satu-
rated :—
j? = 7'12 10"20 15-31 26-15 28'55 36'70 48'K<> (51-15 71'97
t = l° 45° 60° 67D 67° 67° (55° 53° 20°
It is exactly the same in the solution of benzene, aniline, and other substances in
molten sulphur. Alexeeff discovered a similar complete intermixture for solutions of
secondary butyl alcohol in water at about 107° ; at lower temperatures the solubility is
not only limited, but between 50° and 70° it is at its minimum, both for solutions of the
alcohpl in water and for water in the alcohol ; and at a temperature of 5° both solutions
exhibit a fresh change in their scale of solubility, so that a solution of the alcohol in
water which is saturated between 5° and 40° will become cloudy when heated to 60°.
In the solution of liquids in liquids, Alexeeff observed a lowering in temperature (an
absorption of heat) and an absence of change in specific heat (calculated for the mixture)
much more frequently than had been done by previous observers. As regards his affir-
"N WATEB AND ITS COMPOUNDS 75
most cases solid substances in dissolving in water evolve heat, notwith-
standing the passage into a liquid state, which indicates so considerable
an evolution of ( + ) heat in the act of combination with water that it
exceeds the absorption of ( — ) heat dependent on the passage into a
liquid state. Thus, for instance, calcium chloride, CaCl,, magnesium
sulphate, ^IgSO,, and many other salts in dissolving evolve heat ; for
example, 60 grams of magnesium sulphate evolves about 10,000 units
of heat. Therefore, in the solution of solid bodies there is produced
either a cooling 2G or a heating 27 effect, according to the difference of
the reacting affinities. When they are considerable — that is, when
water is with difficulty separated from the resultant solution, and only
with a rise of temperature (such substances absorb water) — then
much heat is evolved in the process of solution, just as in many
reactions of direct combination, and therefore a considerable heating of
the solution is observed. "Of such a kind, for instance, is the solution
matioii (in the sense of a mechanical and not a chemical representation of solutions) that
substances in solutions preserve their physical states (as gases, liquids, or solids), it is
very doubtful, for it would necessitate admitting the presence of ice in water or its
vapour. His theory starts from an unsupported hypothesis — which is, however, held by
many — that the sizes (weights) of the molecules of one and the same substance are very
different in different physical states. At present the weight of gaseous molecules is
determined from the freezing of solutions (see later), and therefore it must either be
admitted that solutions contain gaseous molecules or else that the weight of liquid
molecules is the same as that of gaseous molecules, which is far simpler and more
probable.
From what has been said above, it will be clear that even in so very simple a case as
solution, it is impossible to calculate the heat emitted by chemical action alone, and that
the chemical process cannot be separated from the physical and mechanical.
16 The cooling effect produced in the solution of solids (and also in the expansion of
gases and in evaporation) is applied to the production of low temperatures. Ammo-
nium nitrate is very often used for this purpose ; in dissolving in water it absorbs 77
units of heat per each part by weight. On evaporating the solution thus formed, the
solid salt is re-obtained. The application of the various freezing mixtures is based on
the same principle. Snow or broken ice frequently enters into the composition of these
mi. rin res, advantage being taken of its latent heat of fusion in order to obtain the
lowest possible temperature (without altering the pressure or employing heat, as in other
methods of obtaining a low temperature). For laboratory work recourse is most often
had to a mixture of three parts of snow and one part of common salt, which causes the
temperature to fall from 0° to - 21° C. Potassium thiocyanate, KCNS, mixed with water
(f by weight of the salt) gives a still lower temperature. By mixing ten parts of crystal-
line calcium chloride, CaCl2,6H2O, with seven parts of water, the temperature may even
fall from 0° to - 55°.
27 The heat which is evolved in solution, or even iu the dilution of solutions, is also
sometimes made use of in practice. Thus caustic soda (NaHO), in dissolving or on the
addition of water to a strong solution of it, evolves so much heat that it can replace fuel.
In a steam boiler, which has been previously heated to the boiling point, another boiler
is placed containing caustic soda, and the exhaust steam is made to pass through the
latter ; the formation of steam then goes on for a somewhat long period of time without
any other heating. Norton makes use of this for smokeless street boilers.
of Milphuric arid (nil of vitro! II 2S( ) ,), and of can-tic soda (Xall< >),
Are., in water.-"
Solution exhibits a reverse reaction : that is to >av. if the water be
expelled from a solution, the sulistanee originally taken is re-ol)t;iiiied.
Unt it must l»e borne in mind that the expulsion of the water taken for
-olution is not accomplished with equal facility throughout, because
watei1 lias different decrees of chemical atHnit v for the substance di>-
-.olved. Thus, if a solution of sulphuric acid, which mixes with water
in all proportions, lie heated, it will be found that very different
decrees of heat are required to expel the water. When it is in a lari^e
• i-,,,.; :, ,n. ,,-,,!-,, t',i.- -r. '.d.-t ri-c ..f temperature, corre^pi'iiiN witli the f.u-niMt i<m <•[ :\
?nh\dr,tte II SO ,.-jH < ) ~:',-\ p.c. H S( ) , . wliii-li \ erv lil<ely repeals it -elf in a similar
f, ,,.,,, I,, other -.nhiticii-. alllHiu.L'h all t he phenomena i of cunt raeti.ni, ••vnlut imi of heat, ami
,-;„. ,,i temperature- are \rry c..mplex ami are depemlenl 1.11 many <-i rcii mst a nces. One
.....Mil, I tliinl;. h.-u. ••..-!•. ju. i-iii-.' from the alm\e example-, that all other influcnc<-s are
(,.(.l,ler in their .1, -tii. ii than ehemiral attraction, especially when it is so er,nsi(leral>le as
I,, t'Ai-«-ii -ulphunr aci.l ainl v.ater.
ON WATKK AND ITS r< >M I'< H'NDS 77
excess, water already begins to come off at a temperature slightly
above 100°, but if it be in but a small proportion there is such a
relation between it and the sulphuric acid that at 120°, 150°, 200°, and
even at 300°, water is still held by the sulphuric acid. The bond
between the remaining quantity of water and the sulphuric acid is
evidently stronger than the bond between the sulphuric acid and the
excess of water. The force acting in solutions is consequently of
different intensity, starting from so feeble an attraction that the proper-
ties of water — as, for instance, its power of evaporation — are but very
little changed, and ending with cases of strong attraction between the
water and the substance dissolved in or chemically combined with it. In
consideration of the very important signification of the phenomena, and
of the cases of the breaking up of solutions with separation of water
or of the substance dissolved from them, we shall further discuss them
separately, after having acquainted ourselves with certain peculiarities
of the solution of gases and of solid bodies.
The solubility of gases, which is usually measured by the volume
cf gas29 (at 0° and 760 mm. pressure) per 100 volumes of water, varies
not only with the nature of the gas (and also of the solvent), and
with the temperature, but also with the pressure, because gases them-
selves change their volumes considerably with the pressure. As might
be expected, (1) gases which are easily liquefied (by pressure and cold) are
more soluble than those which are liquefied with difficulty. Thus, in
100 volumes of water there dissolve at 0° and 760 mm. only two volumes
of hydrogen, three volumes of carbonic oxide, four volumes of oxygen,
&c., for these are gases which are liquefied with difficulty ; whilst
-p If a volume of gas v be measured under a pressure of // mm. of mercury (at 0°)
and at a temperature t° Centigrade, then, according to the laws of Boyle, Mariotte, and
of Gay-Lussac combined, its volume at 0° and 760 mm. will equal the product of v into
760 divided by the product of h into l + at°, where a is the co-efficient of expansion of
gases, which is equal to 0'00367. The weight of the gas will be equal to its volume at
0° and 760 mm. multiplied by its density referred to air and by the weight of one volume
of air at 0° and 760 mm. The weight of one litre of air under these conditions being =
1-293 grams. If the density of the gas be given in relation to hydrogen this must be
divided by 14'4 to bring it in relation to air. If the gas be measured when saturated
with aqueous vapour, then it must be reduced to the volume and weight of the gas when
dry, according to the rules given in Note 1. If the pressure be determined by a
column of mercury having a temperature /, then by dividing the height of the column by
1 + 0'00018£ the corresponding height at 0° is obtained. If the gas be enclosed in a
tube in which a liquid stands above the level of the mercury, the height of the column
of the liquid being = H and its density = D, then the gas will be under a pressure which
TTITv
is equal to the barometric pressure less , where 13'59 is the density of mercury. By
13' 59
these methods the quantity of a gas is determined, and its observed volume reduced to
normal conditions or to parts by weight. The physical data concerning vapours and
gases must be continually kept in sight in dealing with and measuring gases. The student
must become perfectly familiar with the calculations relating to gases.
78 PRINCIPLES OF CHEMISTRY
there dissolve 180 volumes of carbonic anhydride, 130 of nitrous oxide,
and 437 of sulphurous anhydride, for these are gases which are rather
easily liquefied. (2) The solubility of a gas is diminished by heating,
which is easy to understand from what has been said previously — that
the elasticity of a gas becomes greater as it is further removed from a
liquid state. Thus 100 volumes of water at 0° dissolve 2-5 volumes of
air, and at 20° only 1*7 volumes. For this reason cold water, when
brought into a warm room, parts with a portion of the gas dissolved in
it.30 (3) The quantity of the gas dissolved varies directly with the pres-
sure. This rule is called the late of Henry and Dalton, and is applicable
to those gases which are little soluble in water. Therefore a gas is
separated from its solution in water in a vacuum, and water saturated
with a gas under great pressure parts with it if the pressure be dimi-
nished. Thus many mineral springs are saturated underground with
carbonic anhydride under the great pressure of the column of water
above them. On coming to the surface, the water of these springs
boils and foams in giving up the excess of dissolved gas. Sparkling
wines and aerated waters are saturated under pressure with the same
gas. They hold the gas so long as they are in a well-corked vessel.
When the cork is removed and the liquid comes in contact with air at
a less pressure, part of the gas, unable to remain in solution at a lesser
pressure, is separated as foam with the hissing sound familiar to all.
It must be remarked that the law of Henry and Dal ton belongs to the
class of approximate laws, like the laws of gases (Gay-Lussac's and
Mariotte's) and many others — that is, it expresses only a portion of a
complex phenomenon, the limit towards which the phenomenon aims.
The matter is rendered complicated from the influence of the degree of
solubility and of affinity of the dissolved gas for water. Gases which
are little soluble — for instance, hydrogen, oxygen, and nitrogen — follow
the law of Henry and Dalton the most closely. Carbonic anhydride
exhibits a decided deviation from the law, as is seen from the determi-
nations of Wroblewski (1882). He showed that at 0° a cubic centi-
metre of water absorbs 1 -8 cubic centimetres of the gas under a pressure
of one atmosphere ; under 10 atmospheres, 16 cubic centimetres (and
not 18, as it should be according to the law) ; under 20 atmospheres,
•~° According to Bunsen, 100 volumes of water under a pressure of one atmosphere
absorb the following volumes of gas (measured at 0° and 7(50 mm.) : —
123 456 7 89 10
0° 4-11 2-03 1-93 179-7 3'3 ISO'S 437'1 688-6 5'4 104960
10° 3-25 1-61 1-93 118-5 2'6 92'0 358-G 513-8 4'4 81280
20° 2-84 1-40 1-93 90'1 2"3 67'0 290'5 362'2 3'5 65400
I, oxygen ; 2, nitrogen : 3, hydrogen ; 4, carbonic anhydride ; 5, carbonic oxide; 6, nitrous oxide;
7, hydrogen sulphide ; 8, sulphurous anhydride ; 9. marsh gas ; 10, ammonia.
ON WATKi; AM) ITS Co.MI'orNliS
79
26'6 cubic centimetres (instead of 36) ; and under 30 atmospheres, 33'7
cubic centimetres.31 However, as the researches of Sechenoff show,
the absorption of carbonic anhydride within certain limits of change
of pressure, and at the ordinary temperature, by water — and even by
solutions of salts which are not chemically changed by it, or do not
form compounds with it — very closely follows the law of Henry and
Dalton, so that the chemical bond between this gas and water is so
feeble that the breaking up of the solution with separation of the gas
is accomplished by a decrease of pressure alone.32 The case is different
if a considerable affinity exists between the dissolved gas and water.
Then it might even be expected that the gas would not be entirely
separated from water in a vacuum, as should be the case with gases
according to the law of Henry and Dalton. Such gases — and, in
general, all which are very soluble — exhibit a distinct deviation from
the law of Henry and Dalton. As examples, ammonia and hydro-
chloric acid gas may be taken. The former is separated by boiling and
decrease of pressure, while the latter is not, but they both deviate dis-
tinctly from the law.
1 'iv-sure in mm.
of mercury
Ammonia dissolved
in 10(1 grams of
water at 0°
Hydrochloric acid
gas dissolved in 100
trnmis of water at 0°
( I rams
Grama
100
28*0
65-7
500
M~> 78-2
1,000
112-6 85-6
1,500
165-6
It will be remarked, for instance, from this table that whilst the pres-
31 These figures show that the co-efficient of solubility decreases with an increase of
pressure, notwithstanding that the carbonic anhydride approaches a liquid state. And,
indeed, liquefied carbonic anhydride does not intermix with water, and does not exhibit a
rapid increase in solubility at its temperature of liquefaction. This indicates, in the first
place, that solution does not consist in liquefaction, and in the second place that the solu-
liility of a substance is determined by a peculiar attraction of water for the substance
dissolving. Wroblewski even considers it possible to admit that a dissolved gas retains
its properties as a gas. This he deduces from his experiments, which showed that the
rate of diffusion of gases in a solvent is, for gases of different densities, inversely propor-
tional to the square roots of their densities, just as the velocities of movement of gaseous
molecules (see Note 34 on p. 80). Wroblewski showed the affinity of water, H2O, for carbonib
anhydride, COo, from the fact that on expanding moist compressed carbonic anhydride
(compressed atO° under a pressure of 10 atmospheres) he obtained (a fall in temperature
takes place from the expansion) a very unstable definite crystalline compound, COo + 8H2O.
32 As, according to the researches of Roscoe and his collaborators, ammonia exhibits
a considerable deviation at low temperatures from the law of Henry and Dalton, whilst
at 100° the deviation is small, it would appear that the dissociating influence of tem-
perature tells on all gaseous solutions ; that is, at high temperatures, the solutions of
all gases will follow the law, and at lower temperatures there will in all cases be a
deviation from it.
80 PRINCIPLES OF CHEMISTRY
sure increased 10 times, the solubility of ammonia only increased 4!,
times.
A number of examples of such cases of the absorption of gases
by liquids might be cited which do not in any \\;iv, even approximately,
agree with the laws of solubility. Thus, for instance, carbonic anhy-
dride is absorbed by a solution of caustic potash in water, and if there
be sufficient caustic potash it is not separated from the solution by a
decrease of pressure. This is a case of more intimate chemical com-
bination. A less completely studied, but similar and clearly chemical,
correlation appears in certain cases of the solution of gases by water,
and we shall afterwards take an example of this in the solution of
hydrogen iodide ; but first we will stop to consider a remarkable appli-
cation of the law of Henry and Dalton33 in the case of the solution of
a mixture of two gases, and this we must do all the more because the
phenomena which then take place cannot be foreseen without a clear
theoretical representation of the nature of gases.34
53 The ratio between the pressure and the amount of gas dissolved was discovered by
Henry in 1805, and Dalton in 1807 pointed out the adaptability of this law to cases of
gaseous mixtures, introducing the conception of partial pressures which is absolutely
necessary for a right comprehension of Dalton's law. The conception of partial pressures
essentially enters into that of the diffusion of vapours in gases (footnote 1) ; for the
pressure of damp air is equal to the sum of the pressures of dry air and of the aqueous
vapour in it, and it is admitted as a sequence to Dalton's law that evaporation in dry
air takes place as in a vacuum. It is, however, necessary to remark that the volume of
a mixture of two gases (or vapours) is only approximately equal to the sum of the volumes
of its constituents (the same, naturally, also refers to their pressures) — that is to say, in
mixing gases a change of volume occurs, which, although small, is quite apparent when
carefully measured. For instance, in 1888 Brown showed that on mixing various volumes
of sulphurous anhydride (SO2) with carbonic anhydride (at equal pressures of 7<>0 mm.
and equal temperatures) a decrease of pressure of 3'9 millimetres of mercury was
observed The possibility of a chemical action in similar mixtures is evident from the
fact that equal volumes of sulphurous and carbonic anhydrides at — 19° form, according
to Pictet's researches in 1888, a liquid having the signs of a chemical compound, or a
solution similar to that given when sulphurous anhydride and water combine into an
unstable chemical whole.
51 The origin of the now generally-accepted kinetic theory of gases, according to
which they are animated by a rapid progressive movement, is very ancient (Bernoulli and
others in the last century had already developed a similar representation), but it was
only generally accepted after the mechanical theory of heat had been established, and
after the work of Krb'nig (1855), and especially after its mathematical side had been
worked out by Clausius and Maxwell. The pressure, elasticity, diffusion, and internal
friction of gases, the laws of Boyle, Mariotte, and of Gay-Lussac and Avogadro-Gerhardt
are not only explained (deduced) by the kinetic theory of gases, but also expressed with
perfect exactitude ; thus, for example, the magnitude of the internal friction of different
gases was foretold with exactitude by Maxwell, by applying the theory of probabilities to
the concussion of gaseous particles. The kinetic theory of ga^es must therefore be con-
sidered as one of the most brilliant acquisitions of the latter half of the present century.
The velocity of the progressive movement of the gaseous particles of a gas, one cubic
centimetre of which weighs d grams, is found, according to the theory, to be equal to
the square root of the product of SpDg divided by d, where p is the pre>suiv under which
oN WATER AND ITS COMPOUNDS 81
Tin l<nr of partial pressures is as follows : — The solubility of gases
in intermixture with each other does not depend on the influence of
the total pressure acting on the mixture, but on the influence of that
portion of the total pressure which is due to the volume of each given gas
in the mixture. Thus, for instance, if oxygen and carbonic anhydride
were mixed in equal volumes and exerted a pressure of 760 millimetres,
(1 is determined expressed in centimetres of the mercury column, D the weight of a cubic
centimetre of mercury in grams (-0 = 13*59,^ = 76, consequently the normal pressure =
l.o:i:; grams on a sq. c. m.), and g the acceleration of gravity in centimetres (^ = 980'5,
at the sea level and long. 45°, = 981'92 at St. Petersburg ; in general it varies with the
longitude and altitude of the locality). Therefore, at 0° the velocity of hydrogen is 1,843,
and of oxygen 461, metres per second. This is the average velocity, and (according to-
Maxwell and others) it is probable that the velocities of individual particles are different,
that is, they occur in, as it were, different conditions of temperature, which is very im-
portant to take into consideration in the investigation of many phenomena proper to-
matter. It is evident from the above determination of the velocity of gases, that
different gases at the same temperature and pressure have average velocities, which are
inversely proportional to the square roots of their densities ; this is also shown by direct
experiment on the flow of gases through a fine orifice, or through a porous wall. This
<l/Nfii>iitt<ii- n -J ncit if of flow for different gases is frequently taken advantage of in
chemical researches (see Chap. II. and also Chap. VII. on the law of Avogadro-Gerhardt)
in order to separate two gases having different densities and velocities. The difference
of the velocity of flow of gases also determines the phenomenon cited in the following
footnote for demonstrating the existence of an internal movement in gases.
If for a certain mass of a gas which fully and exactly follows the laws of Mariotte
and Gay-Lussac the temperature t and the pressure p be simultaneously changed, then
the entire change would be expressed by the equation pr = C (1 + at), or, what is the
same, pv = RT, where T-t + 273 and C and R are constants which vary not only with the
units of measurement but with the nature of the gas and its mass. But as there are
discrepancies from both the fundamental laws of gases (which will be spoken of in the
following chapter), and as, on the one hand, a certain attraction between the gaseous
molecules must be admitted, and on the other hand it must be acknowledged that the
gaseous molecules themselves occupy a portion of a space, therefore for ordinary gases,,
within any considerable variation of pressure and temperature, recourse should be had
to Van der Waal's formula —
(p + ^r) (v—p) = R (I— at)
where a is the true co-efficient of expansion of gases. As the actual co-efficient of ex-
pansion of air at the atmospheric pressure and between temperatures of 0° and 100° =
0'00367, when determined from the change of pressure (according to Kegnault's data)
and when determined from the change of volume = 0'00368 (according to Mendeleeff and
Kayander), and for other gases there is a discrepancy, although not a large one (see the
following chapter), which is considerable at high pressures and for great densities, there-
fore that co-efficient of expansion should be taken which all gases have at low pressures.
This quantity is approximately 0'00367.
The formula of Van der Waal has an especially important significance in the case
of the passage of a gas into a liquid state, because the fundamental properties of both
pi M-. and liquids are equally well expressed, although only in their general features,
by it.
The further development of the questions referring to the subjects here touched on,
which are of especial interest for the theories of solutions, must be looked for in special
memoirs and works on theoretical and physical chemistry. A small part of this subject
will be partially considered in the footnotes of the following chapter.
VOL. I.
82 PRINCIPLES OF CHEMISTRY
then water would dissolve so much of each of these gases as would be
dissolved if each separately exerted a pressure of half an atmosphere,
and in this case, at 0° one cubic centimetre of water would dissolve
0-02 cubic centimetre of oxygen and 0*90 cubic centimetre of carbonic
anhydride. If the pressure of a gaseous mixture equals It, and in u
volumes of the mixture there be a volumes of a given gas, then its
solution will proceed as though this gas were dissolved under a pres-
sure - . That portion of the pressure under influence of which the
solution proceeds is termed the ' partial ' pressure.
In order to represent to oneself the cause of the law of partial
pressures, an explanation must be given of the fundamental properties
of gases. Gases are elastic and disperse in all directions. All that is
known of gases obliges one to think that these fundamental properties
of gases are due to a rapid progressive movement, in all directions,
which is proper to their smallest particles (molecules).35 These mole-
cules in impinging against an obstacle produce a pressure. The greater
the number of molecules impinging against an obstacle in a given time,
the greater the pressure. The pressure of a separate gas or of a gaseous
mixture depends on the sum of the pressures of all the molecules, on
the number of blows in a unit of time on a unit of surface, and on the
mass and velocity (or the vis viva) of the impinging molecules. To the
obstacle all molecules (although different in nature) are alike ; it is
submitted to a pressure due to the sum of their vis viva. But, in a
chemical action such as the solution of gases, on the contrary, the
50 Although the actual movement of gaseous molecules, which is acknowledged by the
kinetic theory of gases, cannot be seen, yet its existence may be rendered evident by
taking advantage of the difference in the velocities which undoubtedly belongs to
different gases which are of different densities under equal pressures. The molecules of a
light gas must move more rapidly than the molecules of a heavier gas in order to produce
the same pressure. Let us take, therefore, two gases — hydrogen and air ; the former is
14'4 times lighter than the latter, and hence the molecules of hydrogen must move almost
four times more quickly than air (more exactly 3'8, according to the formula given in the
preceding footnote). Consequently, if air occurs inside a porous cylinder and hydrogen
outside, then in a given time the volume of hydrogen which succeeds in entering the
cylinder will be greater than the volume of air leaving the cylinder, and therefore the
pressure inside the cylinder will rise until the gaseous mixture (of air and hydrogen)
attains an equal density both inside and outside the cylinder. If now the experiment
be reversed and air surround the cylinder, and hydrogen be inside the cylinder, then more
gas will leave the cylinder than enters it, and hence the pressure inside the cylinder
will be diminished. In these considerations we have replaced the idea of the number
of molecules by the idea of volumes. We shall learn afterwards that equal volumes
of different gases contain an equal number of molecules (the law of Avogadro-Ger-
hardt), and therefore instead of speaking of the number of molecules we can speak of
the number of volumes. If the cylinder be partially immersed in water the rise and fall
of the pressure can be observed, and consequently the experiment can be rendered self-
evident.
ON WATKII AND ITS COMPOUNDS 83
nature of the impinging molecules plays the most important part. In
impinging against a liquid, a portion of the gas enters into the liquid
itself, and is held by it so long as other gaseous molecules impinge
against the liquid — exert a pressure on it. As regards the solubility of
a given gas, for the number of blows it makes on the surface of a liquid,
it is immaterial whether other molecules of gases impinge side by side
with it or not. Therefore, the solubility of a given gas will be propor-
tional, not to the total pressure of a gaseous mixture, but to that por-
tion of it which is due to the given gas separately. Further, the satura-
tion of a liquid by a gas depends on the fact that the molecules of
gases that have entered into a liquid do not remain at rest in it,
although they enter in a harmonious kind of movement with the mole-
cules of the liquid, and therefore they throw themselves off from the
surface of the liquid (just like its vapour if the liquid be volatile). If
in a unit of time an equal number of molecules penetrate into (leap
into) a liquid and leave (or leap out of) a liquid, it is saturated. It
is a case of mobile equilibrium, and not of rest. Therefore, if the
pressure be diminished, the number of molecules departing from the
liquid will exceed the number of molecules entering into the liquid,
and a fresh state of mobile equilibrium only takes place under a fresh
equality of the number of molecules departing from and entering into
the liquid. Thus are explained the main features of the solution, and
furthermore of that special (chemical) attraction (penetration and har-
monious movement) of a gas for a liquid, which determines both the
measure of solubility and the degree of stability of the solutions pro-
duced.
The consequences of the law of partial pressures are exceedingly
numerous and important. All liquids in nature are in contact with the
atmosphere. The atmosphere, as we shall afterwards see more fully,
consists of an intermixture of gases, chiefly four in number — oxygen,
nitrogen, carbonic anhydride, and aqueous vapour. 100 volumes of
air contain, approximately, 78 volumes of nitrogen, and about 21
volumes of oxygen ; the quantity of carbonic anhydride, by volume,
does not exceed 0'05. Under ordinary circumstances, the quantity of
aqueous vapour is much greater, but it varies with the moisture of the
atmosphere. Consequently, the solution of nitrogen in a liquid in
contact with the atmosphere will proceed under a partial pressure equal
to j7(*0 x 760 mm. if the atmospheric pressure equal 760 mm. ; con-
sequently, under a pressure of 600 mm. of mercury, the solution of
oxygen will proceed under a partial pressure of about 160 mm., and
the solution of carbonic anhydride only under the very small pressure
of 0'4 mm. Therefore, although the amount of nitrogen in air is
G2
84 PRINCIPLES OF CHEMISTRY
large, yet, as the solubility of oxygen in water is twice that of the
nitrogen in water, the proportion of oxygen dissolved in water will be
greater than its proportion in air. It is easy to calculate what quantity
of each of the gases will be contained in water, and we will take the
most simple case, and calculate what quantity of oxygen, nitrogen, and
carbonic anhydride will be dissolved from air having the above com-
position at 0° and 760 mm. pressure. Under a pressure of 760 mm. 1
cubic centimetre of water dissolves 0*0203 cubic centimetre of nitrogen,
or under the partial pressure of 600 mm. it will dissolve 0*0203 x Jg#,
or 0*0160 cubic centimetre ; of oxygen 0*041 1 x 1 •: ", or 0*0086 cubic cen-
0*4
timetre ; of carbonic anhydride 1*8 x~ - or 0*00095 cubic centimetre;
760
consequently, 100 cubic centimetres of water will contain at 0° altogether
2*55 cubic centimetres of atmospheric gases, and 100 volumes of air
dissolved in water will contain about 62 p.c. of nitrogen, 34 p.c. of
oxygen, and 4 p.c. of carbonic anhydride. The water of rivers, wells,
<tc., usually contains more carbonic anhydride. This proceeds from
the oxidation of organic substances falling in the water. The amount
of oxygen, however, dissolved in water appears to be actually about ^
the dissolved gases, whilst air contains only 1 of it by volume. .
According to the law of partial pressures, whatever gas be dissolved in
water will be expelled from the solution in an atmosphere of another gas.
This depends on the fact that gases dissolved in water escape from it
in a vacuum, because the pressure is nil. An atmosphere of another
gas acts like a vacuum on a gas dissolved in water. Separation then
proceeds, because the molecules of the dissolved gas no longer impinge
upon the liquid, are not dissolved in it, and those previously held in solu-
tion depart from the liquid in virtue of their elasticity.3'"' For the same
3(5 Here there may be, properly speaking, two cases : either the atmosphere surround-
ing the solution may be limited, or it may be proportionally so vast as to be unlimited,
like the earth's atmosphere. If a gaseous solution be brought into an atmosphere of
another gas which is limited — for instance, as in a closed vessel — then a portion of the
gas held in solution will be expelled, and thus pass over into the atmosphere surrounding
the solution, and will evince its partial pressure. Let us imagine that water saturated
with carbonic anhydride at 0° and under the ordinary pressure be brought into an
atmosphere of a gas which is not absorbed by water; for instance, that 10 c.c.
of an aqueous solution of carbonic anhydride be introduced into a vessel holding
10 c.c of such a gas. The solution will contain 18 c.c of carbonic anhydride. The
expulsion of this gas goes on until a state of equilibrium is arrived at. The liquid
will then contain a certain amount of carbonic anhydride, which is retained under
the partial pressure of that gas which has been expelled. Now, how much gas will
remain in the liquid and how much will pass over into the surrounding atmosphere ?
In order to solve this problem, let us suppose that x cubic centimetres of carbonic
anhydride are retained in the solution. It is evident that the amount of carbonic anhy-
dride which passed over into the surrounding atmosphere will be 18 — a", and the total
volume of gas will be 10 + 18 — a: or 28 — # cubic centimetres. The partial pressure under
ON WATER AND ITS O >.M P< )TNDS 85
reason a gas may be entirely expelled from a gaseous solution by
boiling — at least, in many cases when it does not form particularly stable
compounds with water. In fact the surface of the boiling liquid will
be occupied by aqueous vapour, and therefore all the pressure acting
on the gas will belong to the aqueous vapour. Consequently, the partial
pressure of the dissolved gas will be very inconsiderable. For this, and
for no other reason, a yas separates from a solution on boiling the liquid
holding it. At the boiling point of water the solubility of gases in
water is still sufficiently great for a considerable quantity of a gas to
remain in solution. The gas dissolved in the liquid is carried away,
together with the aqueous vapour ; if boiling be continued for a long
time, then in the end all the gas will be separated.37
which the carbonic anhydride is then dissolved will be (supposing that the common
JQ ~
pressure remains constant the whole time) equal to OQ_~> consequently there is not in
solution 18 c.c of carbonic anhydride (as would be the case were the partial pressure
•equal to the atmospheric pressure), but only 18 2Q_ , which is equal to x, and conse-
•I Q ~
quently we obtain the equation 18 ou_ =#> hence # = 8'69. Again, where the atmo-
sphere into which the gaseous solution is introduced is not only that of another gas but also
unlimited, then the gas dissolved will, on passing over from the solution, diffuse itself
through this atmosphere, and from its limitedness produce an infinitely small pressure
in the unlimited atmosphere. Consequently, no gas can be retained in solution under
this infinitely small pressure, and it will be entirely expelled from the solution. For
this reason water saturated with a gas which is not contained in air, will be entirely de-
prived of the dissolved gas if left exposed to air. Water also passes off from a solution
into the atmosphere, and it is evident that there might be such a case as a constant
proportion between the quantity of water vaporised and the quantity of a gas expelled
from a solution, so that not the gas alone, but the entire gaseous solution, would pass off.
A similar case is exhibited in solutions which are not decomposed by heat (such as those
of hydrogen chloride and iodide), as will afterwards be considered.
37 However, in those cases when the variation of the co-efficient of solubility with the
temperature is not sufficiently great, and when a known quantity of aqueous vapour
and of the gas passes off from a solution at the boiling point, an atmosphere may be
obtained having the same composition as the liquid itself. In this case the amount of
gas passing over into such an atmosphere will not be greater than that held by the
liquid, and therefore such a gaseous solution will distil over without change, and without
altering its composition during the whole period of boiling or distillation. The solution
will then represent, like a solution of hydriodic acid in water, a liquid which is not
changed by distillation, while the pressure under which this distillation takes place re-
mains constant. Thus in all its aspects solution presents gradations from the most feeble
affinities to examples of intimate chemical combination. The amount of heat evolved in
the solution of equal volumes of different gases is in distinct relation with these variations
of stability and solubility of different gases. 22 '3 litres of the following gases (at 700
mm. pressure) evolve the following number of (gram) units of heat in dissolving in a
large mass of water ; carbonic anhydride 5,600, sulphurous anhydride 7,700, ammonia
8,800, hydrochloric acid 17,400, and hydriodic acid 19,400. The two last-named gases,
which are not expelled from their solution by boiling, evolve approximately twice as
much heat as such gases as ammonia, which are separated from their solutions by boiling,
whilst gases which are only slightly soluble evolve less heat than the latter gases.
86 PRINCIPLES OF CHEMISTRY
It is_ evident that the conception of the partial pressures of gases
should not only be applied to the formation of solutions, but also to all
cases of chemical action of gases. Especially numerous are its appli-
cations to the physiology of respiration, for in these cases it is only the
oxygen of the atmosphere that acts.38
The solution of solids, whilst depending only in a small mea-
sure on the pressure under which solution takes place (because solids
and liquids are almost incompressible), is very clearly dependent on
the temperature. In the great majority of cases the solubility of
solids in water increases with the temperature ; and further, the
rapidity of solution increases also. The latter is determined by the
rapidity of diffusion of the solution formed into the remainder of the
water. The solution of a solid in water, although it is as with gases,
a physical passage into a liquid state, is determined, however, by its
chemical affinity for water ; which is particularly clear from the fact
that in solution there occurs a diminution in volume, a change in the
boiling point of water, a change in the tension of its vapour, in. the
freezing point, and in many similar properties. Were solution a physical,
and not a chemical, phenomenon, it would naturally be accompanied
by an increase and not by a diminution of volume, because generally in
melting a solid increases in volume (its density diminishes). Con-
traction is the usual phenomenon accompanying solution, and takes
place even in the addition of solutions to water,39 and in the solution
58 Among the numerous researches concerning this subject, certain results obtained
by Paul Bert are cited in Chapter III., and here we will point out that Prof. Sechenoff,
in his researches on the absorption of gases by liquids, very fully investigated the
phenomena of the solution of carbonic anhydride in solutions of various salts, and
arrived at many important results, which showed that, on the one hand, in the solution
of carbonic anhydride in solutions of salts on which it is capable of acting chemically (for
example, sodium carbonate, borax, ordinary sodium phosphate), there is not only an
increase of solubility, but also a distinct deviation from the law of Henry and Dalton ;
and, on the other hand, that solutions of salts which are not acted on by carbonic anhy-
dride (for example, the chlorides, nitrates, and sulphates) absorb less of it, by reason of
the competition of the already dissolved salt, and follow the law of Henry and Dalton,.
but all the same show undoubted signs of a chemical action between the salt, water, and
carbonic anhydride. Sulphuric acid (whose co-efficient of absorption is 92 vols. per 100),
when diluted with water, absorbs less and less carbonic anhydride, until the hydrate
H2SO4,H2O (co-eff. of absorption then equals 66 vols.) is formed ; then on further
addition of water the solubility again rises until a solution of 100 p.c. of water ia
obtained.
39 Kremers made this observation in the following simple form : — He took a narrow-
necked flask, with a mark on the narrow part (like that on a litre flask which is used for
accurately measuring liquids), poured water into it, and then inserted a funnel, having a
fine tube which reached to the bottom of the flask. Through this funnel he carefully
poured a solution of any salt, and (having removed the funnel) allowed the liquid to
attain a definite temperature (in a water bath) ; he then filled the flask up to the mark
with water. In this manner two layers of liquid were obtained, the heavy saline solution
ON WATER AND ITS COMPOUNDS 87
of liquids in water,40 just as happens in the combination of substances
when evidently new substances are produced.41 The contraction which
takes place in solution is, however, very small, a fact which depends on
the small compressibility of solids and liquids, and on the insignificance
of the compressing force acting in solution.42 The change of volume
which takes place in the solution of solids and liquids, or the altera-
tion in specific gravity 43 corresponding with it, depends on peculiari-
ties of the dissolving substances, and of water, and, in the majority
of cases, is not proportional to the quantity of the substance dis-
below and water above. The flask was then shaken in order to accelerate diffusion, and
it was observed that the volume became less if the temperature remained constant.
This can be proved by calculation, if the specific gravity of the solutions and water be
known. Thus at 15° one c.c. of a 20 p.c. solution of common salt weighs 1'1500 grams,
hence 100 grams occupy a volume of 86'96 c.c. As the sp.gr. of water at 15° = 0'99916,
therefore 100 grains of water occupy a volume of 100'OB c.c. The sum of the volumes is
187'04 c.c. On mixing, 200 grams of a 10 p.c. solution are obtained. Its specific gravity is
1-0725 (at 15° and referred to water at its maximum density), hence the 200 grams will
occupy a volume of 186'48 c.c. The contraction is consequently equal to 0'56 c.c.
40 The contractions produced in the case of the solution of sulphuric acid in water
are shown in the diagram Fig. 17 (page 7.6). Their maximum is 10' 1 c.c. per 100 c.c. of
the solution formed. A maximum contraction of 4'15 at 0°, 3'78 at 15°, and 3'50 at 30°,
takes place in the solution of 46 parts by weight of anhydrous alcohol in 54 parts of
water. This signifies that if, at 0°, 46 parts by weight of alcohol be taken per 54 parts by
weight of water, then the sum of their separate volumes will be 104'15, and after mixing
their total volume will be 100.
41 This subject will be considered later in this work, and we shall then see that the
contraction produced in reactions of combination (of solids or liquids) is very variable
in its amount, and that there are, although very rare, reactions of combination in which
contraction does not take place, or when an increase of volume is produced.
4- The compressibility of solutions of common salt is less, according to Grassi, than
that of water. At 18° the compression of water per million volumes =48 vols. for a
pressure of one atmosphere ; for a 15 p.c. solution of common salt it is 82, and for a
24 p.c. solution 26 vols. Similar determinations were made by Brown (1887) for saturated
solutions of sal ammoniac (38 vols.), alum (46 vols.), common salt (27 vols.), and sodium
sulphate at + 1°, when the compressibility of water =47 per million volumes. This inves-
tigator also showed that substances which dissolve with an evolution of heat and with an
increase in volume (as, for instance, sal-ammoniac) are partially separated from their
saturated solutions by an increase of pressure (this experiment was especially convincing
in the case of sal-ammoniac), whilst the solubility of substances which dissolve with an
absorption of heat or diminution in volume increases, although very slightly, with an
increase of pressure. Sorby observed the same phenomenon with common salt (1863).
43 The most trustworthy data relating to the variation of the specific gravity of
solutions with a change of their composition and temperature, are collected and discussed
in my work cited in footnote 19. The practical (for the amount of a substance in
solution is determined by the aid of the specific gravities of solutions, both in works and
in laboratory practice) and the theoretical (for specific gravity can be more accurately
observed than other properties, and because a variation in specific gravity governs the
variation of many other properties) interest of this subject, besides the strict rules and laws
to which it is liable, make one wish that this province of data concerning solutions
may soon be enriched by further observations of as accurate a nature as possible. Their
collection does not present any great difficulty, although requiring much time and
attention.
<SS I'RIXCIPLKS OF rilKMJSTRY
solved,14 showing the existence of a chemical action between the solvent
and the substance dissolved which is of the same nature as in all other
forms of chemical relation.1'
Although an alteration of the external pressure does not usually
decompose solutions of solids, nevertheless the feeble development of
•*' Inasmuch us the decree of change exhibited in many properties on the formation of
solutions, is not large, so. owing to the insuflii-ient ac-curacy of observations, a proportion-
ality between this change and a change of composition may, in a first rough approximation
and especially \vithin narrow limits of change of composition, easily be imagined in cases
where it does not even exist. The conclusion of Michel and Kraft is particularly instruc-
tive in this respect: in lsf>4. on the basis of their incomplete researches, they supposed
the increment of the specific gravity of solutions to be proportional to the increment of
a salt in a given volume of a solution, which is only true for determinations of specific
gravity which are exact to the second decimal place — an accuracy insufficient even for
technical determinations. Accurate measurements do not confirm a proportionality
either in this case or in many others where a ratio has been generally accepted ; as, for
example, for the rotatory power (with respect to the plane of polarisation i of solutions, and
for their capillarity, Arc. Nevertheless, such a method is not only still made use of, but
even has its advantages when applied to solutions within a limited scope — as, for instance,
very weak solutions, and for a first acquaintance with the phenomena accompanying
solution, and also as a means for facilitating the application of mathematical analysis to
the investigation of the phenomenon of solution. Judging by the results obtained in my
researches on the specific' gravity of solutions, I think that in many cases it would be
nearer the truth to take the change of properties as proportional, not to the amount of a
substance dissolved, but, to the product of this quantity and the amount of water in
which it is dissolved; all the more so as many chemical re'ations vary in proportion to
the reacting masses, and a similar ratio has been established for many phenomena of
attraction studied by mechanics. This product is easily arrived at when the quantity of
water in the solutions to be compared is constant, as is shown in investigating the fall of
temperature in the formation of ice (nee footnote 41), p. IK)'.
'•' All the different forms of chemical reaction may be said to take place in the process
of solution, il \ CinnbiiKiiiona between the solvent and the substance dissolved, which
are more or less stable (more or less dissociated). This form of reaction is the most
probable, and is that most often observed. ('2 1 Reactions of substitution or of double,
ih-coiHjioxitiun between the molecules. Thus it may be supposed that in the solution of
sal-ammoniac, XII, Cl. the action of water produces ammonia, NH.,HO, and hydrochloric
acid. HC1. which are dissolved in the water and simultaneously attract each other. As
these solutions and many others do indeed exhibit signs which are sometimes indispu-
table of similar double decompositions (thus solutions of sal-ammoniac yield a certain
amount of ammoniai. it is probable that this form of reaction is more often met with
than is generally thought. (Mi Reactions of ixuHK'nmit or rcylnceiui'iit are also probably
met with in solution, all the more as here molecules of dim-rent kinds come into intimate
contact, and it is very likely that the configuration of the atoms in the molecules under
these influences is somewhat different from what it was in its original and isolated
state. One is led to this supposition especially from observations made on solutions of
substances which rotate the plane of polarisation land observations of this kind are very
sensitive with respect to the atomic structure of molecules), because they show, for
example (according to Schneider, iH.slj, that strong solutions of malic acid rotate the
plane of polarisation to the right, whilst its ammonium salts in all degrees of concentra-
tion rotate the plane of polarisation to the left. (4 1 Reactions of <li-<-<>nij>u.ii(H>n under
the influences of solution are not only rational of themselves, but. have in recent years
been recognised by Arrhenins, Ostwald. and others, particularly on the basis of electro-
lytical determinations. If a portion of the molecules of a solution occur in a condition of
decomposition, the other portion mav occur in a yet more complex state of combination,
ON WATKi; AND ITS COMPOUNDS 89
the chemical atlinitics acting in solutions of solids becomes evident
from those multifarious methods by \vhich their solutions are drcum
jiowd, whether they be saturated or not. On heating (absorption of
heat), on cooling, and by internal forces alone, aqueous solutions in
many cases separate into their components or their definite com-
pounds. The water contained in solutions is removed from them
as vapour, or, by freezing, in the form of ice,46 but the tension of the
rn/iour of water 47 held in solution is less than that of water in a free
just as the velocity of the movement of different gaseous molecules may be far from
being the same (see Note 34, p. 80).
It is, therefore, very probable that the reactions taking place in solution vary both
quantitatively and qualitatively with the mass of water in the solution, and the great
difficulty in arriving at a lasting decision on the question as to the nature of the chemical
relations which take place in the process of solution will be understood, and if besides
this the existence of a physical process, like the sliding between and interpenetration of
two homogeneous liquids, be also recognised in solution, then the complexity of the
problem as to the actual nature of solutions, which is now to the fore, appears in its
true light. However, the efforts which are now being applied to the solution of this
problem are so numerous and of such varied aspect that they will offer the coming
investigators a vast mass of material towards the construction of a complete theory of
solution.
For my part, I think that the study of the physical properties of solutions (and
especially of weak ones) which now reigns, cannot give any fundamental and complete
solution of the problem whatever (although it should add much to both the provinces of
physics and chemistry), but that, parallel with it, should be undertaken the study of the
influence of temperature, and especially of low temperatures, the application to solu-
tions of the mechanical theory of heat, and the comparative study of the chemical pro-
perties of solutions. The beginning of all this is already established, but it is impossible
to consider in so short an exposition of chemistry the further efforts of this kind which
have been made up to the present date.
46 If solutions are regarded as being in a state of dissociation (see footnote 19, p. 64) it
would be expected that they would contain free molecules of water, which form one of the
products of the decomposition of those definite compounds whose formation is the cause
of solution. In separating as ice or vapour, water makes, with a solution, a heteroge-
neous system (made up of substances in different physical states) similar, for instance,
to the formation of a precipitate or volatile substance in reactions of double decom-
position.
47 If the substance dissolved is non-volatile (like salt or sugar), or only slightly volatile,
then the whole of the tension of the vapour given off belongs to the water, but if a
solution of a volatile substance — for instance, a gas or a volatile liquid — evaporates, then
only a proportion of the pressure belongs to the water, and the whole pressure observed
consists of the sum of the pressures of the vapours of the water and of the substance
dissolved. The majority of researches bear on the first case, which will be spoken of
presently, and the observations of D. P. Konovoloff (1881) refer to the second case. He
showed that in the case of two volatile liquids, mutually soluble in each other, forming
two layers of saturated solutions (for example, ether and water, note 20, p. 66), both solu-
tions have an equal vapour tension (in the case in point the tension of both is equal to
481 mm. of mercury at 19'8°). Further, he found that for solutions which are formed
in all proportions, the tension is either greater (solutions of alcohol and water) or lesa
(solutions of formic acid) than that which answers to the rectilinear change (proportional
to the composition) from the tension of water to the tension of the substance dis-
solved ; thus the tension, for example, of a 70 p.c. solution of formic acid is less, at all
state, and M- /• //,/" r<ttnr> <>f //<> t'ormntini, of in from solutions is lower
tli:tn O . I-'urther. both the diminution of vapour tension and the
lowering of tin- freexing point proceed, at lea-t in dilute solutions,
alnm-i in proportion to the amount of a substance dis.-olved. ls Thus,
if ]ier 1 I.H » Drains of water there lie in solution 1 . ">. 1 (J grains of common
salt (Na('l), then at 100 the vapour tension of the solutions decreases
li\" 1. L' 1 , -1.) mm. ot the baromet ne eohimn. a^atn.-t 7'iO mm., or the
vapour tension of water, whilst the free/in^ point.- are — U'">> . — '_M'l .
and — t'rl'J respectively. The above figures '•' are almost proportional
temperature-,, than the ten-ion of water and of formic acid itself. Tim-, in thi- case the
ten-ion of a solution i- never equal to the sum of the tension of the di--ol\ in-- liquid-, us
Kt •jnaiili already showed when he dist in^ui-hed thi- ca-e from that in which a mixture
of liquids, which arc insoluble in cadi other, evaporates. I-'IMHI this it is evident that a.
mutual action occurs in solution, which diiniui>lie- the \apoiir ten-ion- pi-oper to the
indi\idual substances, as would lie expected mi the suppo-itioii of the fonnatiou of com-
pounds ,,(' the di>-ol\iiiL: >ul)-tance- in -olutioii-. l)ecau-e the ela-ticity then alway-
'" Thi- amount i- u-ually exprt-ssed liy the wei-ht of the -ul.-tance dissolved per l*»u
pai't- liy weight of water. 1 'rohalily it would l»e lietter to expre-s it \>\ the (|uantity of
the -nli-taiice in a definite volume of the-olution — for instance, in a litre. 1 -peak in
detail of t lie different method- of expres-in^- the competition of solutions in the work
mentioned in note lit. p. f, J.
•' The vai'iatiou of the \apour tension of solutions ha- lieen inve-ti^ated liy many.
Thebe>t kniiwn researches aiv those of Wiillner . ls:,s-isf,e and of Tamilian ilHHTi. Tin-
re>earclie- on the temperature of the formation of ice from various solutions ai'i' ulso
very numerous; Bla^den i_17«wi, Kiidorfl i lsf.1 .. and \^<- L'oppet 1 1*71 1 e-tal>lislied the
1 H"j inn i ]r_r. lint thi- kind of m\'e-ti^'at ion takes its chief inteie>t fi'oin the woi'k ot
Itaoult. lie_run in l^-i> on aijiieous -olution-. and afterward- continued tor -olutions in
\ariou- ot hei- eii~ii\-lro/.en licpiid- - for in-tance. lien/eiie. (', 1 1. 'inch- at I'lU'i I. acetic
acid. ('.,}!,(.)., i It'rT'i . and other-. An especially important intei'ot is attached to these
investigations of llaoult on the lowering of the free/in^ point, l.ecau-e he took >olut ion-
of nmiiv well-kn..wn carlxi
mpolllHls ana discovered a -mi]
molecular weight of the Mili^tances and the temperature of cr\ -talli-at ion of the
solvent, which enaliled this kind of research to he applied to the m\ e-t i-at ion of the
nature of stil>Mtance>. \Ve >hall meet with the ajiplication of Kaoult's reMilts Inter on,
and at pic-enl will only cite the deduction arrived at from the-e re-lilt -. The solution
o! one - liundreilt h part of t hat molecular i: ram u ei- lit \\ Inch corre-pond- \\ ith the formula
of a -ul'-taiice di—olved I for example, Na('l :.s-:,. ( ' .1 1, ( ) ir,. ,\ ,-. in ]oiipart> of a
solvent lower- the free/in^ ]ioint of it - -olution in u a t er if 1 s.'i . iii lien /cue ()' I'.i . and in
acetic acid H-o'.i .or twice an much as with water. And a- in weak >olut ion- t he fall of free/in-
point i- proportional to the amount of the -uli-taiice di--ol\ed. it follow- that the fall of
free/in^' point for all other -olution- may lie calculated from tin- rule. So. for in-tance,
the weight which corre-pond- with the formula of acetone, ( '-H, <>. i- ,"•« ; a -oluti 'on-
t a mi i ix •!' \:i. i-i"J-J. .11 id 1 •!".',:> ^ram- of acet • pel- loo ^ram- o] water lorin- ice ' according
to the determination- of heckmaiim at tt'TTd . r:t:;ii . and :) -_!o . ;(iid the.-e ti^ure- -how
that uiih a Milulion containing o-;,> -ram- of acetone per loo ,,| water the fall of the
temperature ,,| the formation ,,f ice will lie d'ls.") , If 1 -o . and d'17'.H. It mu-t lie
remarked that the la\s of projiort iomtlity I.etween the fall of temperature of tlie forma-
tion of ice, and the composition of a solution.i> in general oul\ approximate, and i- only
applicable to ueak -ollll ion-.
We will here remark that the theoretical intere-t o! tlii- Mibjecl was -t ren^t hened
on the disco\er\ of tin connection existili" between the tall o! teli-i ui. the fall of the
ox WATKK AND ITS COMPOUNDS 91
to the amounts of salt in solution (1, 5, and 10 per 100 of water).
Furthermore, it has been shown by experiment that the ratio of the
diminution of vapour tension to the vapour tension of water at different
temperatures in a given solution is an almost constant quantity/"0 and
temperature of the formation of ice, of osmotic pressure (Van't Hoff, note 19), and of the
electrical conductivity <>f solutions, and we will therefore supplement what we have
ul ready said on the subject by some short remarks on the method of investigating the
phenomenon, and on its theretical results.
In order to determine the temperature of the formation of ice (or of crystallisation
of other solvents), a known solution is prepared and poured into a cylindrical vessel
surrounded by a second similar vessel, leaving a layer of air between the two, which,
being a bad conductor, prevents any rapid change of temperature. The bulb of a sensi-
tive and corrected thermometer is immersed in the solution, and also a bent platinum
wire for stirring the solution ; the whole is then cooled (by immersing the apparatus in a
freezing mixture), and the temperature at which ice begins to separate observed. If the
temperature at first falls slightly lower, nevertheless, it becomes constant when ice
begins to form. By then allowing the liquid to get just warm, and then again observing
the temperature of the formation of ice, an exact determination may be arrived at. If
there be a large mass of solution, the formation of the first crystals may be accelerated
by dropping a small lump of ice into the solution already partially over-cooled. This
only imperceptibly changes the composition of the solution. The observation should be
made at the point of formation of only a very small amount of crystals, as otherwise the
composition of the solution will become altered from their separation. Every precaution
must be taken to prevent the access of moisture to the interior of the apparatus, which
might also alter the composition of the solution or properties of the solvent (for instance,
when using acetic acid).
The very great theoretical interest of these observations on the fall of the tempera-
ture of the formation of ice, which are essentially very simple, dates from the time when
Van't Hoff (note 19) showed that their consequences are in complete accord with those
derived from observations on osmotic pressure. These latter showed that a molecular
(expressed by formulae) quantity of a substance evinces an osmotic pressure in a solu-
tion, which is equal to the atmospheric pressure (when i = 1), or which is greater than it
by i times. The magnitude i, determined from osmotic observations on aqueous solutions,
is also obtained from observations on the fall of the temperature of the formation of ice,
if the fall corresponding with a solution containing 1 gram of a substance per 100 parts
water be multiplied by the molecular weight (according to the formula of the substance,
and expressing the weight of a molecule) of the substance dissolved, and divided by
18'5. Thus from the above data for acetone, it is seen that with a solution containing
1 gram, the fall of temperature of the formation of ice equals 0'818°, and after multiply-
ing by the molecular weight (58), and dividing by 18'5, we have i=l. With sugar and
many other substances (among salts, magnesium sulphate, for instance), with carbonic
anhydride, ttc., both methods give a figure which is nearly unity. For potassium and
sodium chlorides, potassium iodide, nitre, and others, i is greater than 1 but less than
2 ; for sulphuric and hydrochoric acids, sodium and calcium nitrates, and others, i is
nearly 2 ; for solutions of barium and magnesium chlorides, potassium carbonate and
dicliromate, i, according to both methods, is greater than 2 but less than 3. The further
investigation of this subject should show whether these conclusions are entirely general,
and would probably explain better than they do now those remarkable correlations
which are arrived at with the present data.
M This fact, which was established by Gay-Lussac, Prinsep, and v. Babo, is confirmed
by the latest observations, and enables us to express not only the fall of tension (p — p')
its.!!, but its ratio to the tension of water (2—£.\. It is to be remarked that in the
V p I
absence of any chemical action, the fall of tension is either very small, or does not
that for every (dilute) solution the rat io bet wee ti the diminution of vapour
tension and of the free/in^- point is also a sufficiently constant quant it v.'1
1 he diminution of the vapour tension of solutions explains the rise
in boiling point through the solution of soli-,i inui- volatile bodies in
water. The temperature of a vapour is the same as that of the solu-
tion from which it is generated, and therefore it follows that the
aqueous vapour ^i\en oil' from a solution will be superheated. A
saturated solution of common salt boils at 1 US 1 .a solution of :'>:$.">
parts of nitre in 100 parts of water at 11-V[I . and a solution of '.\'2~>
parts of potassium chloride in 1(H) parts of water al 17'.1 . if the tempera-
ture of ebullition be determined bv immersing the thermometer bulb in
the liquid itself. This is another proof of the bond which exists between
water and the substance dissolved. And this bond is seen still more
clearly in those cases (for example, in the solution of nitric or formic
acid in \\ater) where the solution boils at a higher temperature than
either water or the volatile substance dissolved in it. For this reason
the solutions of certain u'a>es for instance, hydriodie or hydrochloric
acid boil above 100'.
The separation of ice from solutions •'- explains both the phenome-
non, well known to seamen, that the ice formed from salt water gives
fresh water, and also the fact that by free/ing, just as by evaporation,
a solution is obtained which is richer in salts than before. This is
taken advantage of in cold countries for obtaining a liquor from sea-
water, which is then evaporated for the extraction of .salt.
< >n the removal of part of the water from a solution (hv evaporation
or the separation of ice), there should be obtained a saturated solution,
and then the substance dissolved should separate out. Solutions satu-
rated at a certain temperature should also separate out a corresponding
part of the substance dissolved if thev be reduced, by cooling, '3 to a
. . • at all note :;:'. . and is not proportional to the quantity of the -ul, -lance added. As
lie. the ten-ion - then equal, accordiut,' to the law of Dalton. to the sum of the
ten-ion- oi the -uh-tanees taken. Therefore, liquids which are ins,, ]uhle in each other
i for example, water ,md chloride of earhom pre-eiit a tension eipial to the -urn of their
dual ten-ions, and the'-efore -udi a mixture hoils at a louei temperature than the
': It. in our example, the fall of tension !„• di\ ided 1-y the tei - on of water, a figure is
ol.taim i .\liielii- nearh llir, t iinc- le— than t he magnitude of 1 . • i temperat lire of
M] the application ot the mechanical theory of heat, and is repeated l,\ man\ imesti^'ated
it t
<>.\ VTATKK AND ITS COMPOUNDS 93
temperature at which the water can no longer hold the former quantity
of the substance in solution. If this separation, by cooling a saturated
solution or by evaporation, take place slowly, cryxtal* of the substance
dissolved an- in many cases formed ; and this is the method by which
crystals of soluble salts are usually obtained. Certain solids very
easily separate out from their solutions in perfectly-formed crystals,
which may attain very large dimensions. Such are nickel sulphate,
alum, sodium carbonate, chrome-alum, copper sulphate, potassium ferri-
cvanidc, and a whole series of other salts. The most remarkable circum-
stance in this is that many solids in separating out from an aqueous
solution retain a portion of water, forming crystallised solid substances
which contain water. A portion of the water previously in the solution
remains in the separated crystals. The water which is thus retained
is called the water of crystallisation. Alum, copper sulphate, Glauber's
salt, and magnesium sulphate contain such water, but neither sal-
ammoniac, nor table salt, nor nitre, nor potassium chlorate, nor silver
nitrate, nor sugar, contains any water of crystallisation. One and the
same substance may separate out from a solution with or without water
of crystallisation, according to the temperature at which the crystals are
formed. Thus common salt in crystallising from its solution in water
at the ordinary or a higher temperature does not contain water of
crystallisation. But if its separation from the solution takes place at
a low temperature, namely below —5°, then the crystals contain 38
parts of water in 100 parts. Crystals of the same substance which
separate out at different temperatures may contain different amounts
of water of crystallisation. This proves to us that a solid dissolved in
water may form various compounds with it, differing in their properties
and composition, and capable of appearing in a solid separate form like
many ordinary definite compounds. This is indicated by the numerous
properties and phenomena connected with solutions, and gives reason
for thinking that there exist in solutions themselves such compounds of
note 24), so these substances do not separate from their saturated solutions on cooling
but on heating. Thus a solution of manganese sulphate, saturated at 70°, becomes cloudy
on further heating. The point at which a substance separates from its solution with a
change of temperature gives an easy means of determining the co-efficient of solubility,
and this was taken advantage of by Prof . Alexeeff for determining the solubility of many
substances. The phenomenon and method of observation is here essentially the same
as in the determination of the temperature of formation of ice. If a solution of a sub-
stain •<• which separates out on heating be taken (for example, the sulphate of calcium
or manj_rane>ei. then at a certain fall of temperature ice will separate out from it, and at
a certain rise of temperature the salt will separate out. From this example, and from
general considerations, it is clear that the separation of a substance dissolved from a
solution should present a certain analogy to the separation of ice from a solution. In
both cases, a heterogeneous system of a solid and a liquid is formed from a homogeneous
(liquid) system.
the substance di>solved, and the sohent or compounds similar to them,
only in a liquid partly decomposed form. Kven the <•<>?<>///• nf mlnt'tun*
may often conlirm this opinion. Copper sulphate forms crystals having
a blue colour and containing- water of crystallisation. If the water of
crystallisation be removed by heating the crystals to redness, a colour-
less anhydrous substance is obtained (a white powder). 1'Yom this it
may be seen that the blue colour belongs to the compound of the copper
salt with water. Solutions of copper sulphate are all blue, and con-
sequently "hey contain a compound similar to the compound formed by
the salt with its water of crystallisation. Crystals of cobalt chloride
when dissolved in an anhydrous liquid like alcohol, for instance <_nve
a blue solution, but when they are dissolved in water a red solution is
obtained, ('rystals from the aqueous solution, according to Professor
Potilit/in. contain six times as much "water (CoCl.,,f>H.,< )) for a ijiven
\\ eiu'lit of the salt, as t hose violet crystals (CoCb. II .,< > ) which are formed
by the evaporation of an alcoholic solution.
That solutions contain particular compounds with water is further
shown bv the phenomena of supersaturated solutions, of so-called crvo-
hvdrates. of solutions of certain acids having constant boiling ]>oints.
and the properties of compounds containing water of crystallisation
whose data it i- indispensable to keep in view in tne consideration of
-i i] Ut li i! IS.
The phenomenon of supersaturated solutions consists in the follow-
ing : •< Mi the refrigeration of a saturated solution of certain salts,-'1
if the liuuid be brought tinder certain conditions, the excess of the solid
ina\' -omctiiiies remain in solution and not separate out. A "Teat
number of substances, and especially sodium sulphate, Na._,S(),, or
( Haulier's sab. ea-ilyform supersaturated solution-. If boiling water
be saturated \\ith tin- salt, and the solution be poured ot]' from any
reniainiiiLf undis-olved salt, and. the boiling beinif still continued, the
Vessel holding the solution be \\-ell closed by cot toll wool, or by fusing up
the vcs>cl. or by covering t he sol ut ion with a layer of oil. i hen it will he
found that this saturated solution does not separate out anv (dauber's
^alt whatever on cooling do\\ ii to the ordinary or even to a much
lower temperature : a It lioii^h \\iihout the abo\e precautions a salt
M -pa rate- out on eoolinir. in the form of crystals \\ hidi contain water of
• 'I , ., , ,!< v.hirli c}..irate ..Hi witli \siiti-r ••!' cry-tiilli-iitii.il t'l.nn sii|M-rsiitunitf<l
n-r.,11- p- -i-.ir.-li. •- hasc |.r(.\ci| that -ii| n-r-at urat.'il snlut ii.ns «!M imt
• i ,,| • , i- ,- -.-in ial |in.|.i-rt , -. 'i lie \ ariation M!'
•, M i',,cin.il "ii "i i, ... A ,-.. take |iliici. an Mi-.lin- ti. tlic i.rilimiry
<'N WATKR AND ITS COMPOUNDS 95
<•! •ystallisation to the amount of Na2S04,10H2O — that is, 180 parts of
water for 142 parts of anhydrous salt. The supersaturated solution
may be moved about or shaken inside the vessel holding it, and no
i-ry>tallisatioii will take place; the salt remains in the solution in as
laruv an amount as at. a higher temperature. If the vessel holding
the supersaturated solution be opened and crystals of Glauber's salt be
thrown in, crystallisation suddenly takes place.53 A considerable rise
in temperature is noticed during this rapid separation of crystals, which
is explained by the salt, previously in a liquid state, passing into a solid
state, by which, as is known, latent heat is evolved. This somewhat
resembles the fact that water may be cooled below 0° (even to — 10°) if
it be left at rest, under certain circumstances, and evolves heat in
suddenly crystallising. Although from this point of view there is a
resemblance, yet in reality the phenomenon of supersaturated solutions
is much more complicated. Thus, on cooling, a saturated solution of
Glauber's salt deposits crystals containing Na2SO4,7H2O,56 or 126 parts
65 Inasmuch as air, as has been shown by direct experiment, contains, although in
very small quantities, minute crystals of salts, and among them of sodium sulphate, air
can bring about the crystallisation of a saturated solution of sodium sulphate in an open
vessel, but it has no effect on saturated solutions of certain other saTts ; for example, lead
acetate. According to the observations of De Boisbaudran, Gernez, and others, isomor-
phous salts (analogous in composition) are capable of evoking crystallisation. Thus, a
supersaturated solution of nickel sulphate crystallises by contact with crystals of sul-
phates of other metals analogous to it, such as those of magnesium, cobalt, copper, and
manganese. The crystallisation of a supersaturated solution, brought about by the con-
tact of a minute crystal, starts from it in rays with a definite velocity, and it is evident
that the crystals as they form propagate the crystallisation in definite directions. This
phenomenon recalls the evolution of organisms from germs. An attraction of similar
molecules ensues, and they dispose themselves in definite similar forms.
56 In these days a view is very generally accepted, which regards supersaturated
solutions as homogeneous systems, which pass into heterogeneous systems (composed of
a liquid and a solid substance), in all respects exactly resembling the passage of water
cooled below its freezing point into ice and water, or the passage of crystals of rhombic
sulphur into monoclinic crystals, and of the monoclinic crystals into rhombic. Although
many phenomena of supersaturation are thus clearly understood, yet the spontaneous for-
mation of the unstable hepta-hydrated salt (with 7H2O), in the place of the more stable
deca-hydrated salt (with mol. 10H2O), indicates a property of a saturated solution of sodium
sulphate which obliges one to admit that it has a different structure form an ordinary
solution. Stcherbacheff affirms, on the basis of his researches, that a solution of the
deca-hydrated salt gives, on evaporation, without the aid of heat, the deca-hydrated salt,
whilst after heating above 33° it forms a supersaturated solution and the hepta-hydrated
salt, which gives reason for thinking that the state of salts in supersaturated solutions
is different from that in ordinary solutions. But in order that this view should be
accepted, some signs must be discovered distinguishing solutions (which are, according to
this view, isomeric) containing the hepta-hydrated salt from those containing the deca-
liydrated salt, and all efforts made in this direction (the study of the properties of the
solutions) have given negative results. Further, according to this view, one would expect
that all supersaturated solutions would contain particular forms of crystallohydrates,
ami, although this is possible, yet up to now nothing of the kind has been observed,
96 PRINCIPLES OF CHEMISTRY
of water per 142 parts of anhydrous salt, and not 180 parts of water, as
in the above-mentioned salt. Further, the crystals containing TH2O
are distinguished for their instability ; if they stand in contact not only
with crystals of Na2SO4,10H2O, but with many other substances, they
immediately become opaque, forming a mixture of anhydrous and deca-
hydrated salts. It is evident that between water and a soluble sub-
stance there may be established different kinds of greater or less stable
equilibrium, of which solutions form one aspect/'7
and one must think that the connection with the fusibility of the deca-hydrated salt
(and of all salts which easily give supersaturated solutions and are capable of forming
several crystallohydrates), and with that decomposition (formation of the anhydrous
salt) which the deca-hydrated salt suffers on melting — plays its part here. As some
crystallohydrates of salts (alums, sugar of lead, calcium chloride) melt without
decomposing, whilst others (like Na2SO4,H.2O) are decomposed, then it may be that the
latter are only in a state of equilibrium at a higher temperature than their melting point.
Did experiment show that the hepta-hydrated salt began to crystallise below 33°, and
that then only the crystals grow, then all the data concerning supersaturated solutions of
sodium sulphate could be explained exclusively in the sense of a super-cooling effect.
At present, however, these questions, notwithstanding the mass of research to which
they have been subjected, cannot be considered as fully resolved. It may here be
observed that in melting crystals of the deca-hydrated salt, there is formed, besides
the solid anhydrous salt, a saturated solution giving the hepta-hydrated salt, so that this
passage from the deca- to the hepta-hydrated salt, and the reverse, takes place with the
formation of the anhydrous (or it may be, mono-hydra ted) salt.
The researches of Pickering (1887) on the amount of heat which is evolved in the
solution of hydrous and anhydrous salts at different temperatures, give reason to think
that at a certain temperature no heat will be evolved in the combination with water; that
is, that probably such a combination will not take place. Thus 106 grams (the molecular
weight in grams) of anhydrous sodium carbonate, NaoCOj, in dissolving in 7,200 grams
( = 400 H2O) of water, evolve 4,300 calories at 4°, 5,300 at 16°, and 5,850 calories at 25° (in
other cases the heat evolved in solution also increases with a rise of temperature). If,
however, the crystallo- hydrate, NaoCO^ , 10H.2O,be taken, then (for the same quantity of
anhydrous salt) an absorption of heat is observed; at 4° -16,250, at 16° — 16,150, and at
25° — 16,300 calories. As in this case a portion of the heat absorbed is due to the fact that
the water of crystallisation taken in a solid state appears in a liquid state, Pickering sub-
tracts the latent heat of liquefaction of ice, and obtains in the given case at 4° -1,700, at
16° — 600, and at 28° -0 calories. From this, the heat of the formation of the crystallo-
hydrate, or the heat evolved by the combination of Na2CO3 with 10H2O, may be
calculated (by subtracting the former quantities from the first). At 4° it is equal to
+ 6,000, at 16° + 5,900, at 25° + 5,850 calories; that is, it distinctly decreases, although
but slightly, with the rise of temperature. It may be that for Na2SO4 at 33° the heats
of the formation of + lOHoO and 7H2O differ but very slightly.
57 Emulsions, like milk, are composed of a solution of glutinous or like substances,
or of oily liquids suspended in a liquid in the form of drops, which arc clearly visible
under a microscope, and form an example of a mechanical formation which resembles
solutions. But the difference from solutions is here evident. There are, however,
solutions which approach very near to emulsions in the facility with which the substance
dissolved separates from them. It has long been known, for example, that a particular
kind of Prussian blue, KFe2(CN)6, dissolves in pure water, but, on the addition of the
smallest quantity of either of a number of salts, it curdles and becomes quite insoluble.
If copper sulphide (CuS), cadmium sulphide (CdS), arsenic sulphide (As2S-), and many
other metallic sulphides, be obtained by a method of double decomposition (by precipi-
ON WATKK AND ITS COMPOUND- 97
Solutions of salts on refrigeration below 0° deposit ice or crys-
tals (\vhich then usually contain water of crystallisation) of the salt
dissolved, and on arriving by this means at a certain degree of con-
centration they solidify in their entire mass. These solidified masses
are termed r>7/o// //'//•<//' '*. My researches on solutions of common salt
(1868) showed that its solution solidifies when it reaches a composition
NaCl + 10H2O (180 parts of water per 58'5 parts of salt), which takes
place at about — 23°. The solidified solution melts at the same temper-
ature, and both the portion melted and the remainder preserve the
above composition. Guthrie (1874-1876) obtained the cryohydrates of
many salts, and he showed that certain of them are formed at com-
paratively low temperatures, whilst others (for instance, corrosive
sublimate, alums, potassium chlorate, and various colloids) are formed
on a slight cooling, to — 2° or even before, and that these contain a
very large amount of water. One can easily imagine that these two
series of cryohydrates differ considerably from each other, but the in-
sufficiency of the existing data58 does not permit of a true judgment
being formed. Nevertheless, in the case of common salt, the cryo-
tating salts of these metals by hydrogen sulphide), and be then carefully washed (by
allowing the precipitate to settle, pouring off the liquid, and again adding sulphuretted
hydrogen water), then, as was shown by Schulze, Spring, Prost, and others, the pre-
viously insoluble sulphides pass into transparent (for mercury, lead, and silver, reddish
brown ; for copper and iron, greenish brown ; for cadmium and indium, yellow ; and for
zinc, colourless) solutions, which may be preserved (the weaker they are the longer they
keep) and even boiled, but which, nevertheless, in time become curdled — that is, settle
in an insoluble form, and then sometimes become crystalline and quite incapable of
re-dissolving. Graham and others observed the power shown by colloids (see note 18) of
forming similar hydrusols or solutions of gelatinous colloids, and, in describing alumina,
and silica, we shall have occasion to speak of such solutions once more.
In the existing state of our knowledge concerning solution, such solutions may be
looked on as a transition between emulsion and ordinary solutions, but no fundamental
judgment can be formed about them until a study has been made of their relations to
ordinary solutions (the solutions of even soluble colloids freeze immediately on cooling
below 0°, and, according to Guthrie, do not form cryohydrates), and to supersaturated
solutions, with which they have certain points in common.
58 Offer (1880) concludes, from his researches on cryohydrates, that they are simple
mixtures of ice and salts, having a constant melting point, just as there are alloys having a
constant point of fusion, arid solutions of liquids with a constant boiling point (see note 60).
This does not, however, explain in what form a salt is contained, for instance, in the
cryohydrate, NaCl + 10H2O. At temperatures above — 10° common salt separates out in
anhydrous crystals, and at temperatures near —10°, in combination with water of
crystallisation, NaCl + 2H2O, and, therefore, it is very improbable that at still lower
temperatures it would separate without water. If the possibility of the solidified cryo-
hydrate containing XaCl + 2H2O and ice be admitted, then it is not clear why one of
these substances does not melt before the other. If alcohol does not extract water from
the solid mass, leaving the salt behind, this does not prove the presence of ice, because
alcohol also takes up water from the crystals of many hydrated substances (for instance,
from NaCl + 2H2O) at about their melting-points. Besides which, a simple observation
on the cryohydrate, NaCl + lOH.^O, shows that with the most careful cooling it does not
VOL. I. H
98 PRINCIPLES OF CHKMJSTKY
hydrate with 10 molecules of water, and in the case of sodium nitrate,
the cryohydrate ~'9 with 7 molecules of water (i.e., 126 parts of water
per 85 of salt) should be accepted as established substances, capable of
passing from a solid to a liquid stare and conversely ; and therefore it
may be thought that in cryohydrates we have solutions which are not
only undecomposable by cold, but also have a definite composition which
would present a fresh case of definite equilibrium between the solvent
and the substance dissolved.
The formation of definite but unstable compounds in the process of
solution becomes evident from the phenomena of a marked decrease of
vapour tension, or from the rise of the temperature of ebullition which
occurs in the solution of certain volatile liquids and gases in water. As
an example, we will take hydriodic acid, HI, a gas which liquefies on
a very considerable reduction of temperature, giving a liquid which
boils at - 20°. A solution of it containing 57 p.c. of hydriodic acid is
distinguished by its great stability. If it be evaporated by heating,
the hydriodic acid volatilises together with the water in the same
proportions as they occur in the solution, so that the gas passes off
together with the aqueous vapour, and therefore such a solution may be
distilled unchanged, for the distillate will contain the same proportion
of hydriodic acid and water as was originally taken. The solution
boils at a higher temperature than water. The physical properties of
the gas and water in this case already disappear ; there is formed a
stable compound between water and the gas, a new substance which
has its definite boiling point. To put it more correctly, this is not the
temperature of ebullition, but the temperature at which the compound
formed decomposes, forming the vapours of the products of dissociation,
which, on cooling, re-combine. The above-described aqueous solution
boils at 127°. Should a less amount of hydriodic acid be dissolved
in water than the above, then, on heating such a solution, water only
will at first be 'distilled over, until the solution attains the above-
mentioned composition ; it will then distil over unaltered. If more
hydriodic acid be passed into such a solution a fresh quantity of the
gas will dissolve, which, however, may be very easily removed. It
must not, however, be thought that those forces which determine the
on the addition of ice deposit ice, which would occur if ice in intermixture with the- salt
were formed on solidification.
I may add with regard to cryohydrates that, in investigating aqueous solutions of
alcohol (note 19), I concluded, on the basis of the specific gravity, that a compound,
C2H6O + 12H2O, existed, and a solution -of this composition completely solidifies on cool-
ing to —20°, forming well-formed crystals, which melt at about —18°, as was shown by
observations made by W. E. Tischenko and myself. This definite compound reminds
one of cryohydrates in many respects.
59 See note 24.
M.\ WATKK AM) ITS COMPOUNDS !M)
formation of ordinary gaseous solutions play no part whatever in the
formation of a solution having a definite boiling point ; that they do
act is shown from the fact that such constant gaseous solutions vary in
their composition under different pressures/'0 Therefore, it is not at
;o For this reason ('the want of entire constancy of the composition of constant boiling
solutions with a ch-inge of pressure) nrmy deny the existence of definite hydrates formed
by volatile snl»st inces — for instance, by hydrochloric acid and water. They generally
argue as follows: If there did exist a constancy of composition, then it would net be
altered by a change of pressure. But the distillation of constant boiling hydrates is un-
doubtedly accompanied (judging by the vapour densities determined by Binean). like the
distillation of sal-ammoniac, sulphuric acid. Arc., by an entire decomposition of the
previous compound — that is, these substances do not exist in a state of vapour, but
their products of decomposition (hydrochloric acid and water) are gases at the tempera-
ture of volatilisation, whi:-h dissolve in the volatilised and condensed liquids ; but the
solubility of gases in liquids depends on the pressure, and, therefore, the composition of
constant boiling solutions may, and even ought to, vary with a change of pressure, and.
further, the smaller the pressure and the lower the temperature of volatilisation, the
more likely is a true compound to be obtained. According to the researches of Koscoe
and Dittmar (1859), the constant boiling solution of hydrochloric acid proved to contain
18 p.c. of hydrochloric acid at a pressure of 3 atmospheres, 20 p.c. at 1 atmosphere,
and 28 p.c. at ^ of an atmosphere. On passing air through the solution until its
composition became constant (i.e., forcing the excess of aqueous vapour or of hydro-
chloric acid to pass away with the air), then acid was obtained containing about
20 p.c. at 100°, about 23 p.c. at 50°, and about 25 p.c. at 0°. From this it is seen
that by decreasing the pressure and lowering the temperature of evaporation one
arrives at the same limit, where the composition should be taken as HC1 + 6H2O, which
requires 25'26 p.c. of hydrochloric acid. Fuming hydrochloric acid contains more than
this.
The most important fact in evidence of the existence of definite compounds in acids
boiling at a constant temperature is the fall of tension. The gas loses its tension, does not
follow the law of Henry and Dalton with a diminution of pressure ; its solution oaly parts
with water ; the vapour tension of a volatile liquid in solution is less than its own or that
of the water combined with it. This loss of tension is a loss of movement brought about
by the action of the attraction existing between the water and the substance dissolved. In
the case already considered, as in the case of formic acid in the researches of D. P.
Konovaloff (note 47), the constant boiling solution corresponds with a minimum tension —
that is, with a boiling point higher than that of either of the component elements. But
there is another case of constant boiling solutions similar to the case of the solution of
propyl alcohol, C.'-H^O, when a solution, undecomposed by distillation, boils at a lower
point than that of the more volatile liquid. However, in this case also, if there be
solution, the possibility cannot be denied of the formation of a definite compound in the
form C-,HsO-fH2O, and the tension of the solution is not equal to the sum of tensions
of the components. There are possible cases of constant boiling mixtures even when there
is no solution nor any loss of tension, and consequently no chemical action, because the
amount of liquids that, are volatilised is determined by the product of the vapour den
into their vapour tensions (Wanklyn), in consequence of which liquids whose boiling
point is above 100° — for instance, turpentine and ethereal oils in general — when distilled
with aqueous vapour, pass over at a temperature below 100°. Consequently, it is not in
the constancy of composition and boiling point (temperature of decomposition) that the
signs of a clear chemical action should be seen in the above-described solutions of acids,
but in the great loss of tension, which completely resemble* the loss of tension ob-
>erved. for instance, in the perfectly-definite combinations of substances with water of
crystallisation (see later, note i'i.">). Sulphuric acid. H..SO,. as we shall learn later, is a!-o
decomposed by distillation, like HC1 + 6H.2O, and exhibits, moreover, all the signs of a
II L'
100 FIUNril'LES OF CHEMISTRY
every, but only at the ordinary, atmospheric pressure that a constant
boiling solution of hydriodic acid will contain 57 p.c. of the gas. At
another pressure the proportion of water and hydriodic acid will be
different. It varies, however, judging from observations made by Roscoe,
very little for considerable variations of pressure. This variation in
composition directly indicates that pressure exerts an influence on the
formation of unstable chemical compounds which are easily dissociated
(with formation of a gas), just as it influences the solution of gases,
only the latter is influenced to a more considerable degree than the
former/'1 Hydrochloric, nitric, and other acids form solutions 1iarin</
definite boiling points, like that of hydriodic acid. They show further
the common property, if containing but a small proportion of water, that
they fume in air. Strong solutions of nitric, hydrochloric, hydriodic,
and other gases are even termed ' fuming acids.' The fuming liquids
contain a definite compound, whose temperature of ebullition (decom-
position) is higher than 100°, and contain also an excess of the volatile
substance dissolved, which (the substance) exhibits a capacity to com-
bine with water and form a hydrate, whose vapour tension is less than
that of aqueous vapour. On evaporating in air, this dissolved substance
meets the atmospheric moisture and forms a visible vapour (fumes) with
it, which consists of the above-mentioned compound. The attraction
or affinity which binds, for instance, hydriodic acid with water is
evinced not only in the evolution of heat and the diminution of vapour
tension (rise of boiling point), but also in many purely chemical rela-
tions. Thus hydriodic acid is produced from iodine and hydrogen
sulphide in the presence of water, but unless water is present this re-
action does not take place/'2
definite chemical compound. The study of the variation of the specific gravities of
solutions as dependent on their composition (see note 19) shows that phenomena of a
similar kind, although of different dimensions, take place in the formation of both H2SO4
from H2O and SO3, and of HC1 + 6H.2O (or of aqueous solutions analogous to it) from HC1
and H20.
61 The essence of the matter may be thus represented. A substance A, either gaseous
or easily volatile, forms with a certain quantity of water, ?zHoO, a definite complex com-
pound AnH^O, which is stable up to a temperature t3 higher than 1003. At this tempera-
ture it is decomposed into two substances, A + H2O. Both boil below t° at the ordinary
pressure, and therefore at t° they distil over and re-combine in the receiver. But if a
part of the substance AnfL^O is decomposed or volatilised, there still remains a portion of
undecomposed liquid in the vessel, which can partially dissolve one of the products of
decomposition, and that in quantity varying with the pressure and temperature, and
therefore the solution at a constant boiling point will have a slightly-different composition
at different pressures.
62 For solutions of hydrochloric acid in water there are still greater differences in
reactions. For instance, strong solutions decompose antimony sulphide (forming hydro-
gen sulphide, H2S), and precipitate common salt from its solutions whilst weak solutions-
do not act thus.
<>N WATKK AND ITS COMPOUNDS 101
.Many compounds containing water of crystallisation are solid sub-
stances (when melted they are already solutions — i.e., liquids) ; further-
more, they are capable of being formed from solutions, as is ice or
aqueous vapour. I propose calling them • •/•//*/'/'/"-// //'//v/A-x. Inasmuch
as the direct presence of ice or aqueous vapour cannot be admitted in
solutions (for these are liquids), although the presence of water may
be, so also there is no basis for acknowledging the presence in solu-
tions of substances in an already -existing state of combination with
water of crystallisation, although they are obtained from solutions as
siu-h.'::{ It is evident that such substances present one of the many
forms of equilibrium between water and a substance dissolved in it.
This form, however, reminds one, in all respects, of solutions — that is,
aqueous compounds which are more or less easily decomposed, with
separation of water and the formation of a less aqueous or an anhydrous
compound. In fact, there are not a few crystals containing water
which lose a part of their water at the ordinary temperature. Of such
a kind, for instance, are the crystals of soda, or sodium carbonate,
which, when separated from an aqueous solution at the ordinary
Temperature, are quite transparent; but when left exposed to air,
lose a portion of their water, becoming opaque, and, in the process,
lose their crystalline appearance, although preserving their original
form. This process of the separation of water at the ordinary tempera-
ture is termed the efflorescence of crystals. Efflorescence takes place
more rapidly under the receiver of an air pump, and especially at a
gentle heat. This breaking up of a crystal is dissociation at the
ordinary temperature. Solutions are decomposed in exactly the same
manner.64 The tension of the aqueous vapour, which is given off from
63 Supersaturated solutions give an excellent proof in this respect. Thus a solution
of copper sulphate generally crystallises in penta-hydrated crystals, CuSC>4 + 5H2O, and
ii-- -uturated solution gives such crystals if it be brought into contact with the minutest
possible crystal of the same kind. But, according to the observations of Lecoq de Bois-
baudran, if a crystal of ferrous sulphate (an isomorphous salt, see note 55), FeSO4 + 7H2O,
be placed in a saturated solution of copper sulphate, then crystals of hepta-hydrated salt,
( 'uSO.j+7H2O, are obtained. It is evident that neither the penta- nor the hepta-hydrated
salt is contained as such in the solution. The solution presents its own particular liquid
form of equilibrium.
64 Efflorescence, like every evaporation, proceeds from the surface. Inside crystals
which have effloresced there is usually found a non-effloresced mass, so that the majority
of effloresced crystals of washing soda show, in their fracture, a transparent nucleus
coated by an effloresced, opaque, powdery mass. It is a remarkable circumstance in this
respect that efflorescence proceeds in a completely regular and uniform manner, so that
the angles and planes of similar crystallographic character effloresce simultaneously,
and i)i this respect the crystalline form determines those part s of crystals where efflo-
rescence starts, and the order in which it continues. In solutions evaporation also
proceeds from the surface, and the first crystals which appear on its reaching the
required degree of saturation are also formed at the surface. After falling to the
bottom the crystals naturally continue to grow (see Chap. X.).
102 PRINCIPLES OF CHK-MJSTKY
crystallo-hydrates is naturally, as with solutions, less than the vapour
tension of water itself '"' at the same temperature, and therefore many
anhydrous salts which are capable of combining with water absorb
aqueous vapour from moist air ; that is, they act like a cold body on
which water is deposited from steam. It is on this that the desiccation
of gases is based, and it must farther be remarked in this respect that
certain substances — for instance, potassium carbonate (Iv3CO3) and
calcium chloride (CaCL>) — not only absorb the water necessary for the
formation of a solid crystalline compound, but also give solutions, or
deliquesce, as it is termed, in moist air. Many crystals do not effloresce
in the least at the ordinary temperature ; for example, copper sulphate,
which may be preserved for an indefinite length of time without efflo-
rescing, but when placed under the receiver of an air pump, if efflores-
cence be once started, it goes on at the ordinary temperature. The
temperature at which the entire separation of water from crystals takes
place varies considerably, not only for different substances but also for
different portions of the contained water. Very often the temperature
at which dissociation begins is very much higher than the boiling point
of water. So, for example, copper sulphate, which contains 36 p.c. of
water, gives up 2 8 '8 p.c. at 100°, and the remaining quantity, namely
7*2 p.c., only at 240°. Alum, out of the 45'5 p.c. of water which it con-
tains, gives up 18-9 p.c. at 100°, 17'7 p.c. at 120°, 7-7 p.c. at 180°, and
1 p.c. at 280°; it only loses the last quantity (1 p.c.) at their temperature
of decomposition. These examples clearly show that the annexation of
water of crystallisation is accompanied by a rather profound, although,
in comparison with instances which we shall consider later, still incon-
65 According to Lescoeur (1883), at 100° a thick solution of barium hydroxide, BaH2O2r
on first depositing crystals (with + H.>O) has a tension of about 630 mm. (instead of 7(50 mm.,
the tension of water), which decreases (because the solution evaporates! to 45 mm., when
all the water is expelled from the crystals, BaH2O.2 + HoO, which are formed, but they
also lose water (dissociate, effloresce at 100°), leaving the hydroxide, BaH^O^, which is per-
fectly undecomposable at 100° — that is, does not part with water. At 73° (the tension of
water is then 265 mm.) a solution, containing 33H.^>O, on crystallising has a tension of
280 mm. ; the crystals BaH2O + 8H2O, which separate out, have a tension of 1(10 mm. ; on
losing water they give BaH2O2 -»-HoO. This substance does not decompose at 7:! . and
therefore its tension =0. Miiller-Erzbach (1884) determines the tension (with reference
to liquid water) by placing similar long tubes with water and tin- substances experi-
mented with in a desiccator, the rate of loss of water giving the relative tension. Thus,
at the ordinary temperature, crystals of sodium phosphate, Na.,HPO j -r 12H.->O, present
a tension of 0*7 compared with water, until they lose 5H2O, then 0'4 until they lose ">HoO
more, and on losing the last equivalent of water the tension falls to 0'04 compared with
water. It is clear that the different molecules of water are held by an unequal force.
Out of the five molecules of water in copper sulphate the two first are comparatively
easily separated, even at the ordinary temperature (but only after several days in a
desiccator, according to Latchinoff) ; the next two are more difficultly separated, and the
last equivalent is held firmly, even at 100°.
ON AVATKK AND ITS COMPOUNDS 103
, <-liaii!_;<' ft its properties. In certain cases the water of crys-
tallisation is only given oft' when the solid form of the substance is
destroyed : when the crystals melt on heating. The crystals are then
said to ma/t in their water of crystallisation. Further, after the separa-
tion of the water, a solid substance remains behind, so that by further
heating it acquires a solid form. This is seen most clearly in crystals
of sugar of lead or lead acetate, which melt in their water of crystalli-
sation at a temperature of 56*25°, and in so doing begin to lose water.
On reaching a temperature of 100° the sugar of lead solidifies, having
lost all its water ; and then at a temperature of 280° the anhydrous and
solidified salt again melts. Sodium acetate (C2H3Na02,3H.,O) melts
at .">8° (but resolidifies only on contact with a crystal, otherwise it may
remain liquid even at 0° ; as the temperature does not change during
solidification, the melted salt can be used for obtaining a constant
temperature of 58°). According to Jeannel, the latent heat of fusion is
about 28 calories, and, according to Pickering, the heat of solution is 35
calories. When melted, this salt boils at 123° — that is, the tension of
the aqueous vapour given off then equals the atmospheric pressure.
It is most important to recognise in respect to the water of crys-
tallisation that its ratio to the quantity of the substance with which it
is combined is always a constant quantity. However often we may
prepare copper sulphate, we shall always find 36*14 p.c. of water in its
crystals, and these crystals always lose four-fifths of their water at
100°, and one-fifth of the whole amount of the water contained remains
in the crystals at 100°, and is only expelled from them at a temperature
of about 240°. The determination of the amount of water of crystal-
lisation is easily made if a weighed quantity of crystals is dried in an
air or other bath. What has been said about crystals of copper sulphate
refers also to crystals of every other substance which contain water of
crystallisation. It is impossible to here increase either the relative
proportion of the salt or of the water, without changing the homo-
geneity of the substance. If once a portion of the water be lost — for
instance, if once efflorescence takes place— a mixture is obtained, and
not a homogeneous substance, namely a mixture of a substance deprived
of water with a substance which has not yet lost water — i.e., decom-
position has already commenced. This constant ratio is an example of
the fact that in chemical compounds the quantity of the component
parts is quite definite ; that is, it is an example of the so-called definite
(•lumical compounds. They may be distinguished from solutions, and
from all other so-called indefinite chemical compounds, in that at least
one, and sometimes both, of the component parts may be added in a
large quantity to an indefinite chemical compound without destroying
104 PRINCIPLES OF CIIK.MJSTRY
its homogeneity, as in solutions, whilst it is impossible to add any one
of the component parts to a definite chemical compound without de-
stroying the homogeneity of the entire mass. Definite chemical com-
pounds only decompose at a certain rise in temperature ; on a lowering
in temperature they do not, at least with very few exceptions, yield
their components like solutions which form ice or compounds with water
of crystallisation. This obliges one to consider that solutions contain
water as water, Mj although it may sometimes be in a very small quan-
tity. Therefore solutions which are capable of entirely solidifying (for
instance, cryohydrates 'and crystallo-hydrates — i.e., compounds with
water of crystallisation which are capable of melting — or the compound
of 84^ parts of sulphuric acid, H2SO4, with hH parts of water, H20,
or H2SO4,H2O, or H4SO^) appear as true definite chemical compounds,
If, then, we imagine such a definite compound in a liquid state, and
admit that it partially decomposes in this state, separating water —
not as ice or vapour (for then the system would be heterogeneous.
including substances in different physical states), but in a liquid form,
when the system will be homogeneous — -then we shall form an idea of
a solution as an unstable, decomposing fluid equilibrium between water
and the substance dissolved. Just as the component elements may be
added to a gaseous mixture without destroying its homogeneity, so both
the solvent may be added to a solution (the solution will then be
obtained diluted, and no longer presenting a definite composition), arid
also the substance dissolved may be added (with a solid and a saturated
solution a supersaturated solution will be obtained), which may, how-
ever, owing to the force of the cohesion of its parts, separate out from
the solution in a crystallised form. In adding the solvent, or the
substance dissolved, without destruction of the homogeneity of the
whole, we altered their relative quantity (the proportion of the acting
masses), by which there will be an alteration, both in the quantity of the
water, forming one of the products of dissociation, and also of the relative
quantity of one or many of the definite compounds between the water
and the substance dissolved. Owing to this change, there occurs an
alteration in the properties of a solution (contraction, change of vapour
tension, &c.) ; not in the sense of a purely mechanical change in the
proportion of the components (as in the intermixture of non reacting
66 Such a phenomenon frequently presents itself in purely chemical action. Km-
instance, let a liquid substance A give, with another liquid substance J3, under the condi-
tions of an experiment, a mere minute quantity of a solid or gaseous substance C. This
small quantity will separate out (pass away from the sphere of action, as Berthollet
expressed it), and the remaining masses of A and I? will again give C ; consequently,
under these conditions, action will go on to the end. Such, it seems to me, is the action
in solutions when they yield ice or vapour indicating the presence of water.
OX WATKK AND ITS COMPOUNDS 105
gases), 1'iit in the sense of an alteration in the quantity of those definite
liquid chemical compounds which are determined by the chemical attrac-
tion between water and the substance dissolved in it, and by their
capacity for forming with it 'liri-rxe compounds,*'" which is seen in the
capacity of one substance to form with water many various crystal I »-
},,/,} rofrx, or compounds with water of crystallisation, showing diverse
and independent properties. From these considerations, solution
ma ;/ l»> regarded as fluid, unstable, definite chemical compounds in a
state of dissociation.™
67 Certain substances are capable of forming only one compound, others several, and
these of the most varied degrees of stability. The compounds of water are instances of
tins kind. In solutions of sulphuric- ;ic-icls (nee note 19), for example, the existence must
l)f acknowledged of several different definite compounds. Many of these have not yet
In 'en obtained in a free state, and it may be that they cannot be obtained in any other
but a liquid form— that is, dissolved; just as there are many undoubted definite com-
pounds which only exist in one physical state. Among the hydrates such instances
occur. The compound CO2 + 8HUO (see note 31), according to Wroblewski, only occurs in
a solid form. Hydrates like H.,S + 1'2H2O (De Forcrand and Villard), HBr + H2O (Rooze-
boom), can only be accepted on the basis of a decrease of tension, but present themselves
as very transient substances, incapable of existing in a stable free state. Even sulphuric
acid, H.iSO4, itself, which undoubtedly is a definite compound, fumes in a liquid form,
evolving the anhydride, SO5 — that is, exhibits a very unstable equilibrium. The crystallo-
hydrates of chlorine, C13 + 8H2O, of hydrogen sulphide, H2S + 12H<>O (it is formed at 0°,
and is completely decomposed at +1°, as then 1 vol. of water only dissolves 4 vols. of
hydrogen sulphide, while at 0'1° it dissolves about 100 vols.), and of many other gases,
are instances of hydrates which are very unstable.
68 Of such a kind are also other indefinite chemical compounds; for example,
metallic alloys. These are solid substances or solidified solutions of metals. They also
contain definite compounds, and may contain an excess of one of the metals. According
to the experiments of Laurie (1888), the alloys of zinc with copper in respect to the electro-
motive force in galvanic batteries behave just like zinc if the proportion of copper in the
alloy does not exceed a certain percentage — that is, until a definite compound is attained
— for then there are yet particles of free zinc ; but if a copper surface be taken, and it be
covered by only one-thousandth part of its area of zinc, then only the zinc will act in a
galvanic battery.
69 According to the above supposition, the condition of solutions in the sense of the
kinetic hypothesis of matter (that is, on the supposition of an internal movement of
molecules and atoms) may be represented in the following form: — In a homogeneous
liquid — for instance, water — the molecules occur in a certain state of, although mobile,
>till stable, equilibrium. When a substance A dissolves in water, its molecules form with
-cveral molecules of water, systems AnHoO, which are so unstable that when surrounded
by molecules of water they decompose and re-form, so that A passes from one mass of
molecules of water to another, and the molecules of water which were at this moment in
harmonious movement with A in the form of the system AnH.^O, in the next instant
may have already succeeded in getting free. The addition of water or of molecules of A
may either only alter the number of free molecules, which in their turn enter into systems
A n\ LO, or they may introduce conditions for the possibility of building up new systems
. 1 ,, H..O, where m is either greater or less than n. If in the solution the relation of the
molecules be the same as in the system AmH<>O, then the addition of fresh molecules of
w;iter or of A would be followed by the formation of new molecules ^4»H2O. The relative
quantity, stability, and composition of these systems or definite compounds will vary in
one or another solution. Such a view of solutions came to me from a most intimate
study of the variation of their specific gravities, to which my book, cited in note 19, is
106 PRINCIPLES OF CHEMISTRY
In regarding solutions from this point of view they come under the
head of those definite compounds which chemistry mainly treats of.70
For this reason we will direct our particular attention to one side of
the subject under consideration, which touches on the essential property
devoted. Definite compounds, Ati^R.^O and Jj^HoO. existing in a tree — for instance,
solid — form, may in certain east's be held in solutions in a dissociated state (although but
partially) ; they are similar in their structure to those definite substances which are.
formed in solutions, but nothing obliges one to think that it is such systems as, for
instance, Na2SO4 + 10H3O, or Na3SO4 + 7H2O, or Xa.>S().4, that are contained in solu-
tions. The comparatively more stable systems J.^jH.,0 which exist in a tree state and
change their physical state must present, although within certain limits of temperature,
an entirely harmonious kind of movement of A with /^H.jO ; the property also and state
of systems AnH^Q and AmU^O, occurring in solutions, is that they are in a liquid
form, although partially dissociated. Substances A}, which give solutions, are distin-
guished by the fact that they can form such unstable systems .l//Ho(), but besides them
they can give other much more stable systems J/^H.,0. Thus ethylene, C'oll,. in dis-
solving in water, probably forms a system C3H4nHoO, which easily splits up into (' .,Il[
and HoO, but it also gives the system of alcohol, CoH.^HoO or C,.HGO, which is compara-
tively stable. Thus oxygen can dissolve in water, and it can combine with it, forming
peroxide of hydrogen. Turpentine, C10H1(3, does not dissolve in water, but it combines
with it in a comparatively stable hydrate. In other words, the chemical structure of
. hydrates, or of the definite compounds which are contained in solutions, is distinguished
not only by its original peculiarities but also by a diversity of stability. A similar struc-
ture to hydrates must be acknowledged in crystallo-hydrates. On melting t hey give actual
(real) solutions. As substances which give crystallo-hydrates, like salts, are capable of
forming a number of diverse hydrates, and as the greater the number of molecules of
water (n) they (J.«H2O) contain the lower is the temperature of their formation, and as
the more easily they decompose the more water they hold, therefore, in the first place,
the isolation of hydrates holding much water existing in aqueous solutions may be
soonest looked for at low temperatures (although, perhaps, in certain cases they cannot
exist in the solid state) ; and secondly, the stability also of such higher hydrates will be
at a minimum under the ordinary circumstances of the occurrence of liquid water.
Hence a further more detailed investigation of cryohydrates (note 58 j may help to the
elucidation of the nature of solutions. But it may be foreseen that certain cryohydrates
will, like metallic alloys, present solidified mixtures of ice with the salts themselves and
their more stable hydrates, and others will be definite compounds.
70 The above representation of solutions, &c., considering them as a particular state
of definite compounds, excludes the independent existence of indefinite compound! ;
by this means that unity of chemical conception is obtained which cannot be arrived
at by admitting the physico-mechanical conception of indefinite compounds. The
gradual transition from typical solutions (as of gases in water, and of weak saline
solutions) to sulphuric acid, and from it and its definite, but yet unstable and liquid,
compounds, to clearly definite compounds, such as salts and their crystallo-hydi -ates,
is so imperceptible, that by denying that solutions pertain to the number of definite
but dissociating compounds, we risk denying the definiteness of the atomic com-
position of such substances as sulphuric acid or of molten crystallo-hydrates. I
repeat, however, that for the present the theory of solutions cannot be considered as
firmly established. The above opinion about them is nothing more than a hypothesis
which endeavours to satisfy those comparatively limited data which we ha\e for the
present about solutions, and of those cases of their transition into definite compounds.
By submitting solutions to the Daltonic conception of atomism, 1 hope that we may not
only attain to a general harmonious chemical doctrine, but also that new motives for
investigation and research will appear in the problem of solutions, which must either
confirm the proposed theory or replace it by another fuller and truer one.
<>N WATF.i; AND ITS CoMI'orNhS 107
of definite compounds as a class to whose number solutions should (or
at least, may) be referred.
\Vr >a\\ above that copper sulphate loses four- fifths of its water at
100° and the remainder at 240°. This means that there are two definite
compounds of water with the anhydrous salt. Washing soda or car-
bonate of sodium, Na2CO3, separates out as crystals, Na2CO3,lCH2O,
containing G'2 '9 p.c. of water by weight, from its solutions at the
ordinary temperature. When a solution of the same salt deposits crystals
at a low temperature, about — 20°, then these crystals contain 71*8 parts
of water per 2S-2 parts of anhydrous salt. Further, the crystals are
obtained together with ice, and are left behind when it melts. If
ordinary soda, with 62-9 p.c. of water, be cautiously melted in its own
water of crystallisation, there remains a salt, in a solid state, containing
only 14-5 p.c. of water, and a liquid is obtained which contains the solu-
tion of a salt which separates out crystals at 34°, which contain 46 p.c.
of water and do not effloresce in air. Lastly, if a supersaturated solu-
tion of soda be prepared, then at temperatures below 8° it deposits
crystals containing 54'3 p.c. of water. Thus there are known ag many
as five compounds of anhydrous soda with water ; and they are dis-
similar in their properties and crystalline form, and even in their
solubility. "We will mention that the greatest amount of water in the
crystals corresponds with a temperature of 20°, and the smallest to the
highest temperature. There is apparently no relation between the
above quantities of water and the salts, but this is only because in each
case the amount of water and anhydrous salt was given in percentages,
but if it be calculated for one and the same quantity of anhydrous salt,
or of water, a great regularity will be observed in the amounts of the
component parts in all these compounds. It appears that for 106 parts
of anhydrous salt in the crystals separated out at — 20° there are 270
parts of water ; in the crystals obtained at 15° there are 180 parts of
water ; in the crystals obtained from a supersaturated solution 126 parts,
in the crystals which separate out at 34°, 90 parts, and the crystals with
the smallest amount of water, 18 parts. On comparing these quantities
of water it may be easily seen that they are in simple proportion to each
other, for they are all divisible by 18, and are in the ratio 15 : 10 : 7 : 5 : 1.
Naturally, direct experiment, however carefully it be conducted, is
hampered with errors, but taking these inevitable errors into con-
sideration, it will be seen that for a given quantity of an anhydrous
substance there occur, in several of its compounds with water,
quantities of water which are in very simple multiple proportion. This
is observed in, and is common to, all definite chemical compounds.
This rule is called the law of multiple proportions. It was discovered
108 PRINCIPLES OF CIIK.Al
by Dal ton, and will be evolved in detail in tiie farther exposition in
this work. For the present we will only state that the law of definite
composition enables the composition of substances to be expressed by
formulae, and the law of multiple proportions permits the application
of co-efficients in a weight of whole numbers, in formulae. Thus the
formula, Na2CO3, 10H.2O, directly shows that in this crystallo-hydrate
there are 180 parts of water to 106 parts by weight of the anhydrous
salt, because the formula of soda, Xa.,C03, directly answers to a weight
of 106, and the formula of water to 18 parts, by weight, which are hnv
taken 10 times.
Tn the above examples of the combinations of water, we saw the
gradually-increasing intensity of the bond between water and a
substance with which it forms a homogeneous compound. There is a
series of such compounds with water, in which the water is held with
very great force, and is only given up at a very high temperature, and
sometimes cannot be separated by any degree of heat without the entire
decomposition of the substance. In these compounds there is generally
. no outward sign whatever of their containing water. A perfectly new
substance is formed from an anhydrous substance and water, in which
sometimes the properties of neither one nor the other substance are
observable. In the majority of cases, a considerable amount of heat is
evolved in the formation of such compounds with water. Sometimes
the heat evolved is so intense that a red heat is produced and light
is emitted. It is hardly to be wondered at, after this, that stable
compounds are formed by such a combination. Their decomposition
requires great heat ; a large amount of work is necessary to separate
them into their component parts. All such compounds are definite,
and, generally, completely and clearly definite. The number of such
definite compounds with water or hydrates, in the narrow sense of the
word, is generally inconsiderable for each anhydrous substance ; in the
greater number of cases, there is formed only one such combination of a
substance with water, one hydrate, having so great a stability. The
water contained in these compounds is often called water of constitution
— i.e., water which enters into the structure or composition of the given
substance. By this it is desired to express, that in other cases tin-
molecules of water are as it were separate from the molecules of that
substance with which it is combined. It is supposed that in the forma-
tion of hydrates this water, even in the smallest particles, forms one
complete whole with the anhydrous substance. Many examples of
the formation of such hydrates might be cited. The most familiar
example in practice is the hydrate of lime, or so-called * slaked ' lime.
Lime is prepared by burning limestone, by which the carbonic anhydride
(>N WATEB AND ITS COMPOUNDS 109
is expelled fnmi it, and there remains a \\liitc stony mass, whi«-h U
dense, compact, and rather tenacious. Lime is usually sold in t In-
form, and hears the name of 'quick' or 'unslaked' lime. If water be
poured over such lime, a great rise in temperature is remarked either
directly, or after a certain time. The whole mass becomes hot, part of
the water is evaporated, the stony mass in absorbing water crumbles into
ponder, and if the water be taken in sufficient quantity and the lime
be pure and well burnt, not a particle of the original stony mass is left —
it all crumbles into powder. If the water be in excess, then naturally
a portion of it remains and forms a solution. This process is called
' slaking ' lime. Slaked lime is used in practice in intermixture with
sand as mortar. Slaked lime is a definite hydrate of lime. If it is
dried at 100° it retains 24-3 p.c. of water. This water can only be
expelled at a temperature above 400°, and then quicklime is re-obtained.
The heat evolved in the combination of lime with water is so intense
that it can set fire to wood, sulphur, gunpowder, &c. Even on mixing
lime with ice the temperature rises to 100°. If lime be melted with a
small quantity of water in the dark, a luminous effect is observed. But,
nevertheless, water may still be separated from this hydrate.71 If
phosphorus be burnt in dry air, a white substance called ' phosphoric
anhydride ' is obtained. It combines with water with such energy, that
the experiment must be conducted with great caution. A red heat is
produced in the formation of the compound, and it is impossible to
separate the water from the resultant hydrate at any temperature.
The hydrate formed by phosphoric anhydride is a substance which is
totally undecomposable into its original component parts by this action
of heat. Almost as energetic a combination occurs when sulphuric
anhydride, SO3, combines with water, forming its hydrate, sulphuric
acid, H2SO,. In both cases definite compounds are produced, but
the latter substance, as a liquid, and capable of decomposition by heat,
giving off the vapour of its volatile anhydride even at the ordinary
temperature, forms an evident link with solutions, and, with an
excess of water, it gives, as a soluble substance, a true solution.
If 80 parts of sulphuric anhydride retain 18 parts of water, this
water cannot be separated from the anhydride, even at a tempera-
ture of 300°. It is only by the addition of phosphoric anhy-
dride, or by a series of chemical transformations, that this water can be
separated from its compound with sulphuric anhydride. Oil of vitriol,
71 In combining with water one part by weight of lime evolves 245 units of heat. A
high temperature is obtained, because the specific heat of the resulting product is small.
Sodium oxide, NaoO, in reacting on water, H2O, and forming caustic soda (sodium
hydroxide), NaHO, evolves 552 units of heat for each part by weight of sodium oxide.
110 PRINCIPLES OF CIIE.M:
or sulphuric acid, is such a compound. If a larger proportion of water
be taken, it will combine with the H2SO, ; for instance, if M parts of
water per 80 parts of sulphuric anhydride be taken, a compound is
formed which crystallises in the cold, and melts at -+- 8°, whilst oil of vitriol
does not solidify at even — 30°. If still more water be taken, the oil of
vitriol will dissolve in the remaining quantity of water. An evolution
of heat takes place, not only on the addition of the water of constitu-
tion, but in a less degree on further additions of water.72 And
therefore there is no distinct boundary, but only a gradual transition,
between those chemical phenomena which are expressed in the forma-
tion of solutions and those which take place in the formation of the
most stable hydrates.73
72 The diagram given in note 28 shows the evolution of heat <m the mixture of
sulphuric acid, or mono-hydrate (HoSO4, i.e. SOs + H-jO), with different quantities of uat ri-
per 100 vols. of the resultant solution. Per 98 grams of sulphuric acid iH..SO(l there are
evolved, on the addition of 18 grams of water, 6,379 units of heat ; with <l<ml>le or three
times the quantity of water 9,418 and 11,187 units of heat, and with an infinitely large
quantity of water 17,860 units of heat, according to the determinations of Thomsen. He
also showed that when HoSO4 is formed from SO3 ( = 80) and H.2O ( = ]KI. 21. ms units of
heat are evolved per 98 parts by weight of the resultant sulphuric acid.
" Thus, for different hydrates the stability with which they hold water is very dis-
similar. Certain hydrates hold water very loosely, and in combining with it evolve
little heat. From other hydrates the water cannot be separated by any degree of heat,
even if they are formed from anhydrides (i.e., anhydrous substances) and water with
little evolution of heat; for instance, acetic anhydride in combining with water evolves an
inconsiderable amount of heat, but the water cannot then be expelled from it. If the
hydrate (acetic acid) formed by this combination be strongly heated it either volatilises
Avithout change, or decomposes into new substances, buj> it does not again yield the original
substances — i.e., the anhydride and water. Here is an instance which gives the reason
for calling the water entering into the composition of the hydrate, water of constitution.
Such, for example, is the water entering into the so-called caustic soda or sodium
hydroxide (see note 71). But there are hydrates which easily part with their water; yet
this water cannot be considered as water of crystallisation, not only because sometimes
such hydrates have no crystalline form, but also because, in perfectly analogous cases,
very stable hydrates are formed, which are capable of particular kinds of chemical
reactions, as we shall learn afterwards. In a word, there is not a distinct boundary
either between the water of hydrates and of crystallisation, or between solution and
hydration.
It must be observed that in separating from an aqueous solution, many substances,
without having a crystalline form, hold water in the same unstable state as in crystals ;
only this water cannot be termed 'water of crystallisation' if the substance which
separates out has no crystalline form. The hydrates of alumina and silica are examples
of such unstable hydrates. If these substances are separated from an aqueous solu-
tion by a chemical process, then they always contain water, and when dried at a
definite temperature, so that the hvgroscopic water may pass off, these substances hold
water in a definite proportion. The formation of a new chemical compound containing
water is here particularly evident, for alumina and silica in an anhydrous stat • have
properties differing from those they show when combined with water, and do not combine
directly with it. The entire series of colloids on separating from water form similar
compounds with it, which have the aspect of solid substances generally, without crystal-
line structure. Besides which, colloids retain water in other different states (srr notes :>7
<)N W.\TKK AND ITS COMPOUNDS 111
\Vt- liave thus considered many aspects and decrees of combination
of various substances with water, or instances of the compounds of
water, when it and other substances form new homogeneous substances,
which in this case will evidently be complex — i.e., made up of different
substances — and although they are homogeneous, yet it must be admitted
that in them there exist those component parts which entered into their
composition, inasmuch as these parts may be re-obtained from them. It
must not be imagined that water really exists in hydrate of lime, any
more than that ice or steam exists in water. When we say that water
occurs in the composition of a certain hydrate, we only wish to point
out that there are chemical transformations in which it is possible to
obtain that hydrate by means of water, and other transformations in
which this water may be separated out from the hydrate. This is all
simply expressed by the words, that water enters into the composition
of this hydrate. If a hydrate be formed by feeble bonds, and be decom-
posed at even the ordinary temperature, then the water appears as one
of the products of dissociation, which in all likelihood is the case in
solutions, and forms the fundamental distinction between them and
other hydrates in which the water is combined with greater stability
and forms a solid substance.
and 18), and most often form gelatinous masses. Water is held in a considerable quan-
tity in solidified glue or boiled albumin. It cannot be expelled from them by pressure ;
hence, in this case there has ensued some kind of combination of the substance with water,
This water, however, is easily separated by drying ; but not the whole of it, a portion
being retained, and this portion belongs, as they say, to the hydrate, although in this
CUM' it is very difficult, if possible, to obtain definite compounds. The absence of any
distinct boundary lines between solutions, crystallo-hydrates, and ordinary hydrates
above referred to, is very clearly seen in such examples.
112 PRINCIPLES OF CHEMISTRY
CHAPTER II
THE COMPOSITION OF WATER, HYDROGEN
THE question now arises, Is not water itself a compound substance ?
Cannot it be formed by the mutual combination of some component
parts ? Cannot it be broken up into its component parts ? There can-
not be the least doubt that if it does split up, and if it is a compound,
then it is a definite one characterised by the stability of the union
between those component parts from which it is formed. From the
fact alone that water passes into all physical states as a homogeneous
whole, without in the least varying in its properties and without split-
ting up into its component parts (neither solutions nor many hydrates
can be distilled— they are split up), we must already conclude, from this
fact alone, that if water is a compound then it is a stable and definite
chemical compound. Like many other great discoveries in the province
of chemistry, it is to the end of the last century that we are indebted
for the important discovery that water is not a simple substance, that
it is composed of two substances like a number of other compound sub-
stances. This was proved by two of the methods by which the com-
pound nature of bodies may be determined as self-evident ; by analysis
and by synthesis — -that is, by a method of the decomposition of water
into, and of the formation of water from, its component parts. In 1781
Cavendish first obtained water by burning hydrogen in oxygen, both of
which gases were already known to him. He concluded from this that
water was composed of two substances. But he did not make more
accurate experiments, which would have shown the relative quantities
of the component parts in water, and which would have determined its
complex nature with certainty. Although his experiments were the
first, and although the conclusion he drew from them was true, yet such
novel ideas as the complex nature of water are not easily recognised so
long as there is no series of researches which entirely and indubitably
proves the truth of such a conclusion. The fundamental experiments
which proved the complexity of water by the method of synthesis, and
of its formation from other substances, were made in 1789 by Monge,
THE ro.MI'OSlTlnN OF WATEU, HYDROGEN' 118
Lavoisier, Fourcroy, and Vauquelin. They obtained four ounces of
water by burning hydrogen, and found that water consists of 15 parts
of hydrogen and 85 parts of oxygen. It was also proved that the
weight of water formed was equal to the sum of the weights of the
component parts entering into its composition ; consequently, water con-
tains all the matter entering into oxygen and hydrogen. The com-
plexity of water was proved in this manner by a method of synthesis.
But we will turn to its analysis — i.e., to its decomposition into its com-
ponent parts. The analysis may be more or less complete. Either
both component parts may be obtained in a separate state, or else
only one is separated and the other is converted into a new compound
in which its amount may be determined by weighing. This will be a
reaction of substitution, such as is often taken advantage of for
analysis. The first analysis of water was thus conducted in 1784 by
Lavoisier and Meusnier. The apparatus they arranged consisted of a
glass retort containing water, naturally purified, and whose weight had
been previously determined. The neck of the retort was inserted into
a porcelain tube, placed inside an oven, and heated to a red heat by
charcoal. Iron filings, which decompose water at a red heat, were
placed inside this tube. The end of the tube was connected with a
worm, for condensing any water which might pass through the tube
unclecomposed. This condensed water was collected in a separate flask.
The gas formed by the decomposition was collected over a water bath
in a bell jar. The aqueous vapour in passing over the red-hot iron was
decomposed, and a gas was formed from it whose weight could be
determined from its volume, its density being known. Besides the
water which passed through the tube unaltered, a certain quantity of
water disappeared in the experiment, and this quantity, in the experi-
ments of Lavoisier and Meusnier, was equal to the weight of the gas
which was collected in the bell jar plus the increase in weight of the
iron filings. Hence the water was decomposed into a gas, which was
collected in the bell jar, and a substance, which combined with the
iron ; consequently, it is composed of these two component parts. This
was the first analysis of water ever made ; but here only one (and not
both) of the gaseous component parts of water was collected separately.
Both the component parts of water can, however, be simultaneously
obtained in a free state. For this purpose the decomposition is brought
about by a galvanic current or by heat, as we shall learn directly.1
1 The first experiments of the synthesis and decomposition of water did not afford,
however, an entirely convincing proof that water was composed of hydrogen and oxygen
only. Davy, who investigated the decomposition of water by the galvanic current,
thought for a long time that, besides the gases, an acid and alkali were also obtained.
VOL. I. I
114 PRINCIPLES OF CHEMISTRY
Water is a bad conductor of electricity— that is, pure water does
not transmit a feeble current ; but if any salt or acid be dissolved in
it, then its conductivity increases, and on the passage of a current
through acidified water it is decomposed into its component parts.
Some sulphuric acid is generally added to the water. By immersing
platinum plates (electrodes) in this water (platinum is chosen because
it is not acted on by acids, whilst many other metals are chemically
acted on by acids), and connecting them with a galvanic battery, it
will be observed that bubbles of gas appear on these plates. The gas
which separates is called detonating gas? because, on approaching a
light, it very easily explodes.3 What takes place is as follows : — First,
the water, by the action of the current, is decomposed into two gases.
The mixture of these gases forms detonating gas. When detonating
gas is brought into contact with an incandescent substance — for instance,
a lighted taper — the gases re-combine, forming water, the combination
being accompanied by a great evolution of heat, and therefore the
vapour of the water formed expands considerably, which it does very
rapidly, and as a consequence of which an explosion takes place -that
is, sound and increase of pressure, and atmospheric commotion, as in
the explosion of gunpowder.
In order to discover what gases are obtained by the decom-
position of water, the gases which separate at each electrode must
be collected separately. For this purpose a V-shaped tube is taken ;
one of its ends is open, and the other fused up. A platinum wire,
terminating inside the tube in a plate, is fused into the closed end ;
He was only convinced of the fact that water contains nothing but hydrogen and oxygen
by a long series of researches, which showed him that the appearance of an acid and
alkali in the decomposition of water proceeds from the presence of impurities (especially
from the presence of ammonium nitrate) in water. A final understanding of the com-
position of water is obtained from the determination of the quantities of the component
parts which enter into its composition. It will be seen from this how many data are
necessary for proving the composition of water — thai is, of the transformations of
which it is capable. What has been said of water refers to all other compounds ; the
investigation of each one, the entire proof of its composition, can only be obtained by the
juxtaposition of a large mass of data referring to it.
2 This gas is collected in a voltameter.
3 In order to observe this explosion without the slighest danger, it is best to proceed
in the following manner. Some soapy water is prepared, so that it easily forms soap
bubbles, and it is poured into an iron trough. In this water, the end of a gas-conducting
tube is immersed. This tube is connected with any suitable apparatus, in which
detonating gas is evolved. Soap bubbles, full of this gas, are then formed. If the
apparatus in which the gas is produced be then removed (otherwise the explosion might
travel into the interior of this apparatus), and a lighted taper be brought to the soap
bubbles, a very sharp explosion takes place. The bubbles should be small to avoid any
danger ; ten, each about the size of a pea, suffice to give a sharp report, like a pistol
shot.
THK COMPOSITION ol- WATKK. HYDROGEN 115
the closed end is entirely filled with water 4 acidified with sulphuric
acid, and another platinum wire, terminating in a plate, is immersed in
the open end. If a current from a galvanic battery be now passed
through the wires an evolution of gases will be observed, and the gas
which is obtained in the open branch mixes with the air, while that in
the closed branch accumulates above the water. As this gas accumu-
lates it displaces the water, which continues to descend in the closed
and ascend into the open branch of the tubes. When the water, in
this way, reaches the top of th'e open end, the passage of the current is
stopped, and the gas which was evolved from one of the electrodes only
is obtained in the apparatus. By this means it is easy to prove that a
particular gas appears at each electrode. If the closed end be con-
nected with the negative pole — i.e., with that joined to the zinc — then
the gas collected in the apparatus is capable of burning. This may be
demonstrated by the following experiment : — The bent tube is taken
off the stand, and its open end stopped up with the thumb and inclined
in such a manner that the gas passes from the closed to the open end.
It will then be found, on applying a lighted lamp or taper, that the
gas burns. This combustible gas is hydrogen. If the same experiment
be carried on with a current passing in the opposite direction — that is,
if the closed end be joined up with the positive pole (i.e., with the
carbon, copper, or platinum), then the gas which is evolved from it does
not burn of itself, but it supports combustion very vigorously, so that
in it a smouldering taper immediately bursts into flame. This gas,
which is collected on the anode or positive pole, is oxygen, which is
obtained, as we saw before (in the Introduction), from mercury oxide
and is contained in air.
Thus in the decomposition of water oxygen appears at the positive
pole and hydrogen at the negative pole, so that detonating gas will be
a mixture of them both. Hydrogen burns in air from the fact that in
doing so it re-forms water, with the oxygen of the air. Detonating
gas explodes from the fact that the hydrogen burns in the oxygen
mixed with it. It is very easy to measure the relative quantities of one
Miid the other gas which are evolved in the decomposition of water.
For this purpose a funnel is taken, whose orifice is closed by a cork
through which two platinum wires pass. These wires are connected
with a battery. Acidified water is poured into the funnel, and a glass
cylinder full of water is placed over the end of each wire (fig. 18).
On passing a current, hydrogen and oxygen collect in these cylinders,
4 In order to fill the tube with water, it is turned up, so that the closed end points
(1 >\v 11 wards and the open end upwards, and water acidified with sulphuric acid is poured
into it.
i 2
116
PRINCIPLES OF CHEMISTKV
and it will easily be seen that two volumes of hydrogen are evolved for
every one volume of oxygen. This signifies that, in decomposing, water
gives two volumes of hydrogen and one volume
of oxygen.
Water is also decomposed into its com-
ponent parts by the action of heat. At the
melting point of silver (960°), and in its pre-
sence, water is decomposed and the oxygen
absorbed by the molten silver, which dissolves
it so long as it is liquid. But directly the
silver solidifies the oxygen is expelled from it.
However, this experiment is not entirely con-
vincing ; it might be thought that in this case
the decomposition of the water did not proceed
from the action of heat, but from the action
of the silver on water — that silver decom-
p°ses water' takins UP the
t s m-
determining the relation be- possible to directly show the decomposition
tween the volumes of hydrogen * . * L
and oxygen. of water by the action of heat, because the
component parts of water, if they remain
together, re-combine with a fall of temperature, and give water back
again. For instance, if steam be passed through a red-hot tube,
whose internal temperature attains 1,000°, then a portion5 of the water
decomposes into its component parts, forming detonating gas. But on
passing into the cooler portions of the apparatus this detonating gas
again reunites and forms water. The hydrogen and oxygen obtained
combine together at a lower temperature.6 Apparently the problem —
5 As water is formed by the combination of oxygen and hydrogen, the reaction evolving
much heat, and as it can also be decomposed, therefore this reaction is a reversible
one (see Introduction), and consequently at a high temperature the decomposition of
water cannot be complete — it is limited by the opposite reaction. Strictly speaking, it is
not known how much water is decomposed at a given temperature, although many efforts
(Bunsen, and others) have been made in various directions to solve this question. Not
knowing the coefficient of expansion, and the specific heat of gases at such high tem-
peratures, renders all calculations (from observations of the pressure on explosion)
doubtful.
6 Grove, about 1840, observed that a platinum wire fused in the flame of detonating
gas — that is, having acquired the temperature of the formation of water — and having
formed a molten drop at its end which fell into water, evolved detonating gas — that
is, decomposed water. It therefore follows that water already decomposes at the tem-
perature of its formation. At that time, this formed a scientific paradox ; this we shall
unravel only with the development of the conceptions of dissociation, introduced into
science by Henri Sainte-Claire Deville, about 1850. These conceptions form an im-
portant epoch in science, and their development is one of the problems of contemporary
chemistry. The essence of the matter is that, at high temperatures, water exists but also
decomposes, just as a volatile liquid, at a certain temperature, exists both as a liquid and
THE COMPOSITION OF WATKK, II VJ>1;< >< i F.N
117
to show the decomposability of water at high temperatures— is un-
attainable. It was considered as such before Henri Sainte- Claire
Deville (in the fifties) introduced the conception of dissociation into
chemistry, as of a change of chemical state resembling evaporation, if
decomposition be likened to boiling, and before he had demonstrated
the decomposability of water by the action of heat in an experiment
which will presently be described. In order to demonstrate clearly the
dissociation of water, or its decomposability by heat, at a temperature
approaching that at which it is formed (as a volatile liquid, at a given
temperature, can be either in a liquid or vaporous condition) it was
necessary to separate the hydrogen from the oxygen at a high tempe-
rature, without allowing the mixture to cool. Deville took advantage
of the difference between the densities of hydrogen and oxygen.
A wide porcelain tube p (fig. 19) is placed in a furnace giving a
FK;. 19. Decomposition of water by the action of heat, and the separation of the hydrogen formed by
its permeating through a porous tube.
strong heat (it should be heated with small pieces of good coke). In
this tube there is inserted a second tube T, of less diameter, and made
of unglazed earthenware and therefore porous. The ends of the tube
are luted to the wide tube, and two tubes, c and c', are inserted into
the ends, as shown in the drawing. With this arrangement it is
possible for a gas to pass into the annular space between the walls
of the two tubes, from whence it can be collected. Steam from
as a vapour. Similarly as a volatile liquid saturates a space, attaining its maximum
tension, so also the products of dissociation have their maximum tension, and once that is
attained decomposition ceases, just as evaporation ceases. Under like conditions, if
the vapour be allowed to escape (and therefore its partial pressure be diminished), evapora-
tion recommences, so also if the products of decomposition be removed, decomposition
again continues. These simple conceptions of dissociation introduce infinitely varied
consequences into the mechanism of chemical reactions, and therefore we shall have
occasion to return to them very often.
118 PRINCIPLES OF CHEMISTRY
a retort or flask is passed through the tube D, into the internal porous
tube T. This steam on entering the red hot space is decomposed into
hydrogen and oxygen. The densities of these gases are very different,
hydrogen being sixteen times lighter than oxygen. Light gases, as \ve
saw above, penetrate through porous surfaces very much more rapidly
than denser gases, and therefore the hydrogen passes through the pores
of the tube into the annular space very much more rapidly than the
oxygen. The hydrogen which separates out into the annular space
can only be collected when this space does not contain any oxygen.
If any air remains in this space, then the hydrogen which separates
out will combine with its oxygen and form water. For this reason a
gas incapable of supporting combustion — for instance, nitrogen— is pre-
viously passed in the annular space. Thus the nitrogen is passed
through the tube c, and the hydrogen, separated from the steam, is
collected through the tube c', and will be partly mixed with nitrogen.
A certain portion of the nitrogen will penetrate through the pores of
the unglazed tube into the interior of the tube T. The oxygen will
remain in this tube, and the volume of the remaining oxygen
will be half that of the volume of hydrogen which separates out from
the annular space. Part of the oxygen will also penetrate through
the pores of the tube ; but, as was said before, a much smaller quan-
tity than the hydrogen, and as the density of oxygen is sixteen
times greater than that of hydrogen, the volume of oxygen which
passes through the porous walls will be four times less than the volume
of hydrogen (the quantities of gases passing through porous walls are
inversely proportional to the square roots of their densities). The
oxygen which separates out into the annular space will combine, at a
certain fall of temperature, with the hydrogen ; but as each volume of
oxygen only requires two volumes of hydrogen, whilst at least four
volumes of hydrogen will pass through the porous walls for every
volume of oxygen that passes, therefore, part of the hydrogen will
remain free, and can be collected from the annular space. A corre-
sponding quantity of oxygen remaining from the decomposition of the
water can be collected from the internal tube.
The decomposition of water is produced much more easily by a
method of substitution, taking advantage of the affinity of substances
for the oxygen or the hydrogen of water. If a substance be added to
water, which takes up the oxygen and replaces the hydrogen — then we
shall obtain the latter gas from the water. Thus with sodium, water
gives hydrogen, and with chlorine, which takes up the hydrogen,
oxygen is obtained.
Hydrogen is evolved from water by many metals, which are capable
TIIK COMPOSITION OK WATKK. HYDROGEN 119
of forming oxides (rusts or earths, as Stahl called them) in air— that is,
which are capable of burning or combining with oxygen. The capacity
of metals for combining with oxygen, and therefore for decomposing
water, or for the evolution of hydrogen, is very dissimilar.7 Among
metals, potassium and sodium have the greatest energy in this respect.
The first occurs in potash, the second in soda. They are both lighter than
water, soft, and easily change in air. By bringing one or the other of
them in contact with water at the ordinary temperature,8 a quantity of
7 In order to demonstrate the difference of the .affinity of oxygen for different
elements, it is enough to compare the amounts of heat which are evolved in their combi-
nation with 16 parts by weight of oxygen ; in the case of sodium (when Na2O is formed,
or 46 parts of Na combine with 16 parts of oxygen, according to Beketoff) 100,000 calories
(or units of heat) are evolved, for hydrogen (when water, H2O, is formed) 69,000 calories,
for iron (when the oxide, FeO, is formed) 69,000, and if the oxide FeoO3 is formed,
64,000 calories, for zinc (ZnO is formed) 86,000 calories, for lead (when PbO is formed)
51,000 calories, for copper (when CuO is formed) 38,000 calories, and for mercury (HgO is
formed) 31,000 calories.
These figures cannot correspond directly with the magnitude of the affinities, for the
physical and mechanical side of the matter is very different in the different cases.
Hydrogen is a gas, and, in combining with oxygen, gives a liquid ; consequently it changes
its physical state, and, in doing so, evolves heat. But zinc and copper are solids, and,
in combining with oxygen, give solid oxides. The oxygen, previously a gas, now passes
into a solid or liquid state, and, therefore, also must have given up its store of heat in
forming oxides. As we shall afterwards see, the degree of contraction (and conse-
quently of mechanical work) was different in the different cases, and therefore the
figures expressing the heat of combination cannot directly depend on the affinities, on
the loss of internal energy previously in the elements. Nevertheless, the figures above
cited correspond, in a certain degree, with the order in which the elements stand hi
respect to their affinity for oxygen, as may be seen from the fact that the mercury oxide,
which evolves the least heat (among the above examples), is the least stable, is easily
decomposed, giving up its oxygen ; whilst sodium, the formation of whose oxide is accom-
panied by the greatest evolution of heat, is able to decompose all the other oxides, taking
up their oxygen. In order to generalise the connection between affinity and the evolu-
tion and the absorption of heat, which is evident in its general features, and was firmly
established by the researches of Favre and Silberman (about 1840), and then of Thomsen
(in Denmark) and Berthelot (in France), many investigators, especially the one last
mentioned, established the law of maximum work. This states that only those chemical
reactions take place of their own accord in which the greatest amount of chemical
(latent, potential) energy is transformed into heat. But, in the first place, we are not
able, judging from what has been said above, to distinguish that heat which corresponds
with purely chemical action from the sum total of the heat observed in a reaction (in the
calorimeter) ; in the second place, there are evidently endothermal reactions which
proceed under the same circumstances as exothermal (carbon burns in the vapour of
sulphur with absorption of heat, whilst in oxygen it evolves heat) ; and, in the third
place, there are reversible reactions, which when taking place in one direction evolve
heat, and when taking place in the opposite direction absorb it ; and, therefore, the
principle of maximum work in its elementary form is not supported by science. But the
subject continues to be developed, and will probably lead to a general law, such as
thermal chemistry does not at present possess.
8 If a piece of metallic sodium be thrown into water, it floats on it (owing to its light-
ness), keeps in a state of continual movement (owing to the evolution of hydrogen on
nil sides), and immediately decomposes the water, evolving hydrogen, which can be
hydrogen, ct irre-pt Hiding with tin- amount of the metal taken, mav be
difectlv obtained. < )nc i_;ram of hvdro^vn. occupying' a \olmiic of
ll'lt* litre-, at () and <<>(>nim.. is evolved jicr -"i!1 plains of i>< itassium,
or •_!•"> u'i'am- of sodium. Tin- phenomenon niav lie observed in tlie
following way :a solution of sodium in mercurv or ' sodium {inuilgain,;
as it is "jeiierallv called i- poured into a vessel containing water, and
ownm' TO its weight sinks to the bottom : the -odium held in the
nierciir\" 1 hen acts on the water like pure sodium, liberating hydrogen.
liif 1 1 n MTU i v doe- not act here, a i id the sail it1 amount of it as \vas taken
for dissolving the sodium i- obtained in the residue. The hydrogen i-
evolved little by little in the form of bubbles, which pa-- through
the liquid.
]>evoiid the hydrogen evolved and a -olid substance, which remains
in solution (it may be obtained by evaporating the iv-ultant solution),
no other products are here obtained. Consequently, from the t wo sub-
stances (water and sodium) taken, the same number (if new substances
(hvdrogen and the substance dis-<il\ed in water) have been obtained,
'fi'oin \\hieh we ma\" conclude that the reaction which here takes place
i- a reaction of double decomposition or of substitution. The sub-
stance- taken were, sodium in a. free -late, and water, which consists of
two ^'ases, hydrogen and oxygen. rl'he pnduct- obtained \\cre.
hvdro^en in the free state and a solid, which i- nothing else lr,;t the so-
ealled dtisticsoda (-odium hvdroxide), \\hich is made ti]» of sodium.
oxvgen. and half of the hydrogen contained in the water. Therefore,
the -ubstitution took" jilace between the hydrou'en and the sodium,
namely half of the hvdrogen in the water was replaced bv the sndiuin.
and was evolved iii a free state. < >n this basis it mav be said that
ca.u si ic soda is nothing else but \\ater. in \\'hich half the hvdrogen
is replaced bv metallic sodium. The reaction which Takes place
Illdll IllilV. lld\Vc\ I'l'. |i-:nl Id ill! i-Xjili - "II -llc.lllil tllr -ndillin -lii k In
• , •, i-l. MIM! IM.-III tu act <nlli«- limited inn - ..I' \\ at"T i icdi;itf]\ adjucciit
- • |. NaUO I'LVIUS witll N;l. \a..n.wll it'll art- till lllf \\atrl'. r\<il\ I II-
It n] . 'I .' i dc-i-i i|il|" i-il ii ill nl Water l'\ MiilillMI lllil\ lie lieltrr (lellltill-trateil.
eati-i f.-ty. iii the full.. wiii.i iiiaiini I'. lulu ;i la-- r\ linder lille.l \\ iih mer
elU-y. • ' ed ill .: I II. • l'( '1 1 1'\ I,,, ill. \\atiT I- lil'-l i 1 1 1 1'ud 1 le. -d . ullirll W 1 1 1. UW i I1LT t ( I its
• , , 1 1 ' • t..|i. and 1 h.'ll a |>iere ui" -uiliiiiii V. l'a|ij.ed in paper is illt nxillced with
. ;, . hi.-l • .. nail.-, iii.il e\.,l\. 1, Mir, ,.•..!,. which riili. ••!-. in tlie cylinder, and
. ;, :, ,| :::, , :'. . :.-.',.. i • i, : i.i-i-n c.miplet. d. 'I'll- -afc-l method ..f inakin-
,.-,p, , • ...- ful|u\\ -. The .udiiiin 'iN am d fi'um the n.i|.litli.i iii \\ Inch
,,•••:. rpi i • . . upper -au/c and hdd l.\ luiveps. «.r el.-e held in
!,,!•< i-p- ,,t the end .1 \\hicli ., mall cupper , , . attached. ,md i- then hdd under
'I'll,. , .. | '.dru^'cli ;_'.,e 1.11 ijlliet i\ 1 it Ilia \ he cullected ill a hell
,11- aii'l thdi IL'liti-l.
THE COMPOSITION <>K WATKK. HYDROGEN 121
may be expressed by the equation : H2O 4- Na=NaHO + H ; the mean-
ing of this is clear from what has been already said.'1
Sodium and potassium act on water at the ordinary temperature.
< >ther heavier metals only act on it with a rise of temperature, and
then not so rapidly or vigorously. Thus magnesium and calcium only
liberate hydrogen from water at its boiling point, and zinc and iron only
at a red heat, whilst a whole series of heavy metals, such as copper, lead,
mercury, silver, gold, and platinum, do not in the least decompose
water at any temperature, and do not replace its hydrogen.
From this it is clear that hydrogen may be obtained by the decom-
position of steam by the action of iron (or zinc) with a rise of tempera-
ture. The experiment is conducted in the following manner : pieces
of iron (tilings, nails. Arc.), are laid in a porcelain tube, which is then
9 This reaction is vigorously exothermal. If a sufficient quantity of water be taken
the whole of the sodium hydroxide, NaHO, formed is dissolved, and about 42,500 units of
heat are evolved per 23 grams of sodium taken. As 40 grams of sodium hydroxide
arc produced, and they in dissolving, judging from direct experiment, evolve about 10,000
calories ; therefore, without an excess of water, and without the formation of a solution,
the reaction Xa + H2O = H + NaHO would evolve about 32,500 calories. We shall after-
wards learn that hydrogen contains in its smallest isolable particles H2 and not H,
and therefore it follows that the reaction should be written thus — 2Na + 2H2O = H2 +
'JXaHO, and it then corresponds with an evolution of heat of 4- 05,000 calories. And as
X. X. Beketoff showed that Na^O, or anhydrous oxide of sodium, forms the hydrate, or
sodium hydroxide (caustic soda), 2NaHO, with water, evolving about 35,500 calories, there-
fore the reaction 2N a + H2O = H2 + NaoO corresponds to 29,500 calories. This quantity
of heat is less than that which is evolved in combining with water, in the formation
of caustic soda, and therefore it is not to be wondered at that the hydrate, NaHO, is always
formed and not the anhydrous substance Na^O. That such a conclusion, which agrees
with facts, is inevitable is also seen from the fact that, according to Beketoff, the anhy-
drous sodium oxide, NaoO, acts directly on hydrogen,with separation of sodium Na^O -t- H =
NaHO + Na. This reaction is accompanied by an evolution of heat equal to about
3,000 calories, because Na2O + H2O gives, as we saw, 35,500 calories and Na + H>O evolves
32,500 calories. However, an opposite reaction also takes place — XaHO + Na = NaoO + H
(both with the aid of heat) — consequently, in this case heat is absorbed. In this we see
an example of calorimetric calculations and the small use of the law of maximum work
for the general phenomena of reversible reactions, to which the case just considered
belongs. But it must be remarked that all reversible reactions evolve or absorb but
little heat, and judging from what has been said in Note 6 (and in Note 25 of Chap. I.),
the reason of the discrepancy between the law of maximum work and reality must
before all be looked for in the fact that we have no means of separating the heat which
corresponds with the purely chemical process from the sum total of the heat observed,
and as the structure of a number of substances is altered by heat alone and also by
contact, we can scarcely hope that the time approaches when such a distinction will be
possible. A heated substance, in point of fact, has no longer the original energy of its
atoms — that is, the act of heating not only alters the store of movement of the molecule^
but also of the atoms forming the molecules, in other words, it makes the beginning of or
preparation for chemical change. From this it must be concluded that thernio-chemistry,
or the study of the heat accompanying chemical transformations, cannot be identified
with chemical mechanics. Thermo-chemical data form a part of it, but they alone
cannot give it.
122 PRINCIPLES OF CHEMISTRY
subjected to a strong heat and steam passed through it. The steam,
coming into contact with the iron, gives up its oxygen to it, and thus
the hydrogen is set free and passes out at the other end of the tube
together with undecomposed steam. This method, which is historically
very significant,10 is practically inconvenient, as it requires a rather
high temperature. Further, this reaction, as a reversible one (a red-
hot mass of iron decomposes a current of steam, forming oxide and
hydrogen ; and a mass of oxide of iron, heated to redness in a stream
of hydrogen, forms iron and steam), does not proceed in virtue of the
comparatively small difference between the affinity of oxygen for iron
(or zinc), and for hydrogen, but only because the hydrogen escapes, as
it is formed, in virtue of its elasticity.11 If the oxygen compounds — that
is, the oxides — which are obtained from the iron or zinc, be able to pass
into solution, then the affinity acting in solution is added, and the
reaction may become non-reversible, and proceed with comparatively
much greater facility.12 As the oxides of iron and zinc, by themselves
10 The composition of water, as we saw above, was determined by passing steam over
red-hot iron ; the same method has been used for making hydrogen for filling balloons.
An oxide having the composition FesC^ is formed in the reaction, so that it is expressed
by the equation 3Fe + 4H^O = Fe5O4+8H. It is very important to remark that this re-
action is reversible. By heating the scoria in a current of hydrogen, water and iron
are obtained. From this it follows, from the principle of chemical equilibria, that if
there be taken iron and hydrogen, and also oxygen, but in such a quantity that
it is insufficient for combination with both substances, then it will divide itself
between the two ; part of it will combine with the iron and the other part with the
hydrogen, but a portion of both will remain in an uncombined state. Here again (see
note 9) the reversibility is connected with the small heat effect, and here again both re-
actions (direct and reverse) proceed at a red heat. But if, in the above-described re-
action, the hydrogen escapes as it is evolved, then its partial pressure does not increase
with its formation, and therefore all the iron can be oxidised by the water, which could
not take place were the iron and water heated to the temperature of reaction in a closed
vessel. In this we see the elements of that influence of mass to which we shall have
occasion to return later.
11 Therefore, if iron and water be placed in a closed space, decomposition of the water
will proceed on heating to the temperature at which the reaction 3Fe + 4H...O = Fe3O4 + 8H
commences ; but it ceases, does not go on to the end, because the conditions for a
reverse reaction are attained, and a state of equilibrium will ensue after the decomposi-
tion of a certain quantity of water. Judging from what has been said in Note 9,
something of the same kind takes place if the iron be replaced by sodium, only
then the mass of the water decomposed will be greater, and equilibrium will ensue,
with the formation of the hydrate, NaHO, and not of anhydrous oxide, NaoO — that is,
the water will remain in the form of hydrate only. With copper and lead there will be
no decomposition, either at the ordinary or at a high temperature, because the affinity of
these metals for oxygen is much less than that of hydrogen.
12 In general, if reversible as well as non-reversible reactions can take place between
substances acting on each other, then, judging by our present knowledge, the non-
reversible reactions take place in the majority of cases, which obliges one to acknowledge
the action, in this case, of comparatively strong affinities. The reaction, Zn + H3SO4 —
H2 + ZnSO4, which takes place in solutions at the ordinary temperature, is scarcely re-
versible under these conditions, but at a certain high temperature it becomes reversible,
T1IK COMPOSITION OF AVATKK. HYDROGEN 123
insoluble in water, are capable of combining with (have an affinity for)
acid oxides (as we shall afterwards fully consider), and form saline and
soluble substances, with acids, or hydrates having acid properties, hence
by the action of such hydrates, or of their aqueous solutions,13 iron
and zinc are able to liberate hydrogen with great ease at the ordinary
temperature — that is, they act on solutions of acids just as sodium acts
on water.14 Sulphuric acid, or oil of vitriol, H2S04, is usually chosen
because at this temperature zinc sulphate and sulphuric acid split up, and the action must
take place between the water and zinc. From the preceding proposition results proceed
which are in some cases verified by experiment. If the action of zinc or iron on a solu-
tion of sulphuric acid presents a non-reversible reaction, then we may by this means
obtain hydrogen in a very compressed state, and compressed hydrogen will not act on
solutions of sulphates of the above-named metals. This is verified in reality as far as
was possible in the experiments to keep up the compression or pressure of the hydro-
gen. Those metals which do not evolve hydrogen with acids, on the contrary, should, at
least at an increase of pressure, be displaced by hydrogen. And in fact Brunner showed
that gaseous hydrogen displaces platinum and palladium from the aqueous solutions of
their chlorine compounds, but not gold, and Beketoff succeeded in showing that silver
and mercury, under a considerable pressure, are separated from the solutions of certain
of their compounds by means of hydrogen. Keaction already commences under H pres-
sure of six atmospheres, if a weak solution of silver sulphate be taken ; with a stronger
solution a much greater pressure is required, however, for the separation of the silver.
15 For the same reason, many metals in acting on solutions of the alkalis displace
hydrogen. Aluminium acts particularly clearly in this respect, because its oxide gives a
soluble compound with alkalis. For the same reason tin, in acting on hydrochloric acid,
evolves hydrogen, and silicon does the same with hydrofluoric acid. It is evident that
in such cases the sum of all the affinities plays a part ; for instance, taking the action of
zinc on sulphuric acid, we have the affinity of zinc for oxygen (forming zinc oxide, ZnO),
the affinity of its oxide for sulphuric anhydride, S05 (forming zinc sulphate, ZnSO4), and
the affinity of the resultant salt, ZnSO4, for water. It is only the first-named affinity that
acts in the reaction between water and the metal, if no account is taken of those forces
(of a physico-mechanical character) which act between the molecules (for instance, the
cohesion between the molecules of the oxide) and those forces (of a chemical character)
which act between the atoms forming the molecule, for instance, between the atoms of
hydrogen giving the molecule H2 containing two atoms. I consider it necessary to
remark, that the hypothesis of the affinity or endeavour of heterogeneous atoms to enter
into a common system and in harmonious movement (i.e., to form a compound molecule)
must inevitably be in accordance with the hypothesis of forces inducing homogeneous
atoms to form complex molecules (for instance, H2), and to build up the latter into
solid or liquid substances, in which the existence of an attraction between the homo-
geneous particles must certainly be admitted. Therefore, those forces which bring about
solution must also be taken into consideration. These are all forces of one and the same
series, and in this may be seen the great difficulties surrounding the study of mole-
cular mechanics and its province — chemical mechanics.
14 The representation given above of the cause of the easy action of iron or zinc on
sulphuric acid, naturally forms a hypothesis which explains only what is observed.
It is only at first sight that this hypothesis exhibits any similarity to the hypothesis of
predisposing affinity which reigned in past times. According to that, it was supposed that
reaction takes place (and hydrogen is evolved) by reason of the affinity for the sulphuric-
acid of the oxide of zinc which might be produced, and that decomposition could
not take place without this. The influence of a force in respect to a substance \shirh lias
not been produced, but which is capable of being formed, is not clear. In the repre-
sentation introduced by me, it is acknowledged that zinc already acts on water by
124 PRINCIPLES OF CHEMISTRY
for this purpose ; from it the hydrogen is displaced by many metals with
incomparably greater facility than directly from water, and such a
displacement is accompanied by the evolution of a large amount of
heat.15 By the action of zinc or iron on sulphuric acid, hydrogen is
evolved, because the metal replaces it. When the hydrogen in sulphuric
acid is replaced by a metal, a substance is obtained which is called a
salt of sulphuric acid or a sulphate. Thus, by the action of zinc on
sulphuric acid, hydrogen and zinc sulphate, ZiiSO^, are obtained.
The latter is a solid substance, soluble in water. In order that the
action of the metal on the acid should go on regularly, and to the end,
it is necessary that the acid should be diluted with water, which dis-
solves the salt as it is formed ; otherwise the salt covers the metal,
and hinders the acid from attacking it. Usually the acid is diluted
with from three to five times its volume of water, and the metal is
covered with this solution. In order that the metal should act
rapidly on the acid, it should present a large surface, so that a maxi-
mum amount of the reacting substances may come into contact in a
given time. For this purpose the zinc is used as strips of sheet zinc,
or in the granulated form (that is, zinc which has been poured from a
certain height, in a molten state, into water). The iron should be in
the form of wire, nails, filings, or cuttings.
The usual method of obtaining hydrogen is as follows :— A certain
quantity of granulated zinc is put into a double- necked, or Woulfe's,
bottle. Into one neck a funnel is placed, reaching to the bottom of
the bottle, so that the liquid poured in may prevent the hydrogen from
itself, even at the ordinary temperature, but that the action is limited by small
masses and only proceeds at the surface. In reality, zinc, in the form of a very
fine powder, or so called ' zinc dust/ is capable of decomposing water with the
formation of oxide (hydrated) and hydrogen. The oxide formed acts 011 sulphuric acid,
water then dissolves the salt produced, and the action continues because one of the
products of the action of water on zinc, zinc oxide, is removed from the surface. One
might naturally imagine that the reaction does not proceed directly between the metal
and water, but between the metal and the acid, but such a simple representation, which
we shall cite afterwards, hides the mechanism of the reaction, and does not permit of its
actual complexity being seen.
15 According to Thomsen the reaction between zinc and a very weak solution of
sulphuric acid evolves about 38,000 calories (zinc sulphate beinjj; formed) per (55 parts
by weight of zinc ; and 56 parts by weight of iron — which combine, like (55 parts by
weight of zinc, with 16 parts by weight of oxygen — evolve about 25,000 calories (forming
ferrous sulphate, FeSO4). Paracelsus observed the action of metals on acids in the
seventeenth century; but it was not until the eighteenth century that Lemery
determined that the gas which is evolved in this action is a particular one which differs
from air and is capable of burning. Even Boyle confused it with air. Cavendish
determined the chief properties of the gas discovered by Paracelsus. At first it was
called 'inflammable air'; later, when it was recognised that in burning it gives water,
it was called hydrogen, from the Greek words for water and generator.
THE COMPOSITION <>F WATKK. HYDROGEN
125
escaping through it. The gas escapes through a special gas-conducting
tube, which is firmly tixctl. by a cork, into the other neck, and which
ids in a water bath (fig. 20), under the orifice of a glass cylinder full
FIG. 20.— Apparatus for the preparation of hydrogen from zinc and sulphuric acid.
of water.16 If sulphuric acid be now poured into the W.oulfe's bottle,
it will soon be seen that bubbles of a gas are evolved, which is hydrogen.
lti As laboratory experiments with gases require a certain preliminary knowledge, we
will describe certain practical methods for the preparation and collection of gases.
When in laboratory practice an intermittent supply of hydrogen (or other gas which is
evolved without the aid of heat) is required the apparatus represented in fig. 21 is the
FIG. 21.— A very convenient apparatus for the preparation of gases obtained without heat. It may
also replace an aspirator or gasometer.
most convenient. It consists of two bottles, having orifices at the bottom, in which
corks with tubes are placed, and these tubes are connected by an india-rubber tube
(sometimes furnished with a spring clamp). Zinc is placed in one bottle, and dilute sul-
1-26
PRINCIPLES OF CHEMISTRY
The first part of the gas evolved should not be collected, as it is
mixed with the air originally in the apparatus. This precaution
phuric acid in the other. The neck of the former is closed by a cork, which is fitted with
a gas-conducting tube with a stop-cock. If the two bottles are put in communication
with each other and the cock be opened, the acid will flow to the zinc and evolve hydro-
gen. If the cock be closed, the hydrogen will force out the acid from the bottle contain-
ing the zinc, and the action will cease. Or the vessel containing the acid may be placed
at a lower level than that containing the zinc, when all the liquid will flow into it, and in
order to start the action^the acid vessel may be placed on a higher level than the other,
and the acid will flow to the zinc. Such an arrangement presents the simplest form of a
continuously-acting apparatus, which is of great use in chemical work. It can also be
employed for collecting gases (as an aspirator or gasometer).
In laboratory practice, however, other forms of apparatus are generally employed for
FIG. 22. — Constant-acting aspirator. The tube d should be long (over 32 feet).
exhausting, collecting, and holding gases. We will here cite the most usual forms. An
aspirator usually consists of a vessel furnished with a stop-cock at the bottom. A stout
THE COMPOSITION <>K WATKK. HYDROGEN
127
should he taken in the preparation of all gases. Time must be allowed
for the gas evolved to displace all the air from the apparatus, Other-
cork, through which u glass tube passes, is fixed into the neck of this vessel. If the
vessel I)*-' tilled u}) with wnter to the cork and the bottom stop-cock be opened, then the
water will run out and draw gas in. For this purpose the glass tube is connected with
the apparatus from which it is desired to pump out or exhaust the gas.
Tib aspirator represented in fig. 22 may be recommended for its continuous
action. It consists of a tube tl which widens out at the top, the lower part being long
and narrow. In the expanded upper portion c, two tubes are sealed ; one, e, for drawing
in the gas. whilst the other, b, is connected to the water supply //*. The amount of water
supplied through the tube b must be less than the amount which can be carried off by
the tube d. Owing to this the water in the tube d will flow through it in cylinders
alternating with cylinders of gas, which will be thus carried away. The gas which is drawn
through may be collected from the end of the tube rf, but this form of pump is usually
employed where the air or gas aspirated is not to be collected. If the tube d is of con-
siderable length, say 40 ft. or more, a very fair vacuum will be produced, the amount of
which is shown by the gauge g ; it is often used for filtering under reduced pressure, as
shown in the figure. If water be replaced by mercury, and the length of the tube d be
greater than 760 mm., the aspirator may be employed as an air-pump, and all the air
may be exhausted from a limited space ; for instance, by connecting g with a hollow
sphere.
Gasholders are often used for collecting and holding gases. They are made of glass,
copper, or tin plate. The usual form is shown in fig. 23. The lower vessel B is made
hermetically tight — i.e., impervious to
gases — and is filled with water. A funnel
is attached to this vessel (on several sup-
ports). The vessel B communicates with
the bottom of the funnel by a stop-cock
b and a tube a, reaching to the bottom of
the vessel B. If water be poured into the
funnel and the stop-cocks a and b opened,
the water will run through a, and the air
escape from the vessel B by b. A glass
tube / runs up the side of the vessel B, with
which it communicates at the top and bot-
tom, and shows the amount of water and
gas the gasholder contains. In order to fill
the gasholder with a gas, it is first filled
with water, the cocks a, b and e are closed,
the nut d unscrewed, and the end of the tube
conducting the gas from the apparatus in
which it is generated is passed into d. As
the gas fills the gasholder, the water runs
out at d. If the pressure of a gas be not
greater than the atmospheric pressure and
it be required to collect it in the gasholder, ^=j
then the cock e is put into communication g
with the space containing the gas. Then, ^H
having opened the orifice d, the gasholder
acts like an aspirator; the gas will pass
through e, and the water run out at d. If
Fig. 23.— Gasholder.
the cocks be closed, the gas collected in the gasholder may be easily preserved and trans-
ported. If it be desired to transfer this gas into another vessel, then a gas-conducting
tube is attached to e, the cock a opened, b and d closed, and then the gas will pass out
at e, owing to its pressure in the apparatus being greater than the atmospheric pressure
l-2s
wi-c in tc-nnu; tin- ci >nil nist il );lit v < »t the hydrogen an explosion inav
occur from the formation of detonating ^as (the mixture of the oxygon
of t he air ^ it h the hydr« i^en ).'"
Hydrogen, which i>> contained in water, and which therefore can
he obtained from it. i> also Contained in manv oilier substances, 's and
may be obtained from Them. A-> examples of this, it may l»e men-
tioned (l)that a mixture of formate of sodium. < ' 1 1 Na< ) .,, and canst ic
soda. Xa 1 1 <>. when heated to redness, forms sodium carbonate. Na.,('< ).,.
and hydrogen. II., : ''•' i'2) tiiat a number of organic -ub.-tances are
deci iinj M isei 1 at a red heat, tormin^ hvdroi^en, amon^ other leases, and
thus it is that hydrogen is contiiined in ordinary liu'htinu' u'as.
( 'harcoal itself liberate- hydrogen from steam at a hiudi tempera -
ture : -" but the reaction which here takes place i- distinguished by a
certain complexity, and will therefore be considered later.
cylinder or tla-k \villi the i;-a>. it i- tilled \vitli water ami inverted in tin1 funnel, and
tin -t ' >| >-<•'>(•]< - // and '/ opened. Then water will run I hrouiih n, and the uM- \\"ill e-cape
from the gasholder into tin- cylinder thromjh //.
'' Wlirn it i-; I'ci pi ircd l<> pri-parc hydroj/pn in larj^v quantities for lillinLT lialloon.-.
cnjipcr vc>sds or \voodcii casks lined \villi lead are eniployed : they are tilled with -crap
i ron . over which dilute sui ] >hu ri<- acid is poured. The hydn i^'eii general ed from a numl per
i it ca >K'~ i> ca rried t hrou^'h lead pi pe- into special ca>l\'- contain 1 1 in' wat er M n order to cool
the ua-1 and lii in order to remove acid fuincsl. To avoid loss of ^a- all the point-
are made hermetic, ill v ti-'ht with a paste of piaster or tar. In order to till his ^i;_'ant ic
halloon lot' i2.".iHio ciihic metre, capacity i. (iit'fard. in ISjS, constriu-tcd a coinjilicale.l
apparatn- f"!' •_;'! \'inur a cont iniious supply of hydrogen, in \\hic!i a mixture of sulphuric
acid and water wa- continually run into \c-sel- containiiiL; iron, and from which the
solution of iron sulphate formed \\a- continually drawn ot't. \\heii coal ua-. ex-
tracted fromcoal. isemjiloyed 1'. >r tilliiiLT halloo us it should lie a> li^lit. or a > rich in hydrogen.
a~ po^-ilde. I''or this reason, only the la-t portion- of the ^as comin.u from the retort-
are collected, and. hcside- thi-. M i- then sometimes passed through red-hot vesseU. in
urdei' to decomjiosc the liydrocarl - a- much as possible; charcoal i- deposited in the
red-hot vosels. and hydros-ell remain- as «rus. Coal -'a- may he yet furt her enriched
in hydro-'en. and couseinient 1\ rendered lighter, hy passing it o\er an ignited mixture ot
charcoal and lime.
i- oi the metal-, only a very few comhine with hydrogen i for exam).le. sodium).
;11,d j_rj\,. >iil»staiices which are ea-il\ <leco)np,,sed. Of the iion-melals, tin- halo-
tahle. wli : ' lho-e ol l.roniine ami iodine are easily dec po.ed. e-pecialh the
Iiydro'_'en compound- ot different coinpo-ition .,,,,] proper! ie-. Imt they are all le-s -talili
1),,,,, '/.ater. The numliei' "t the carl. on compound- of hydrogen is e 'moil-, l.ut tln-ri
hy«lr..'_'i'-n. at a ivd heal.
I , ••,,,, , •. |, . , , , i i : , ei | nation ( 'Na !l( ) NallO CNa .< >.-, II . may \,<
, ... ,.,,., | , . ,. ||,, -,, , , mpo ition o| ,-opp. r , arhoiiate or mercurx oxid.
,.r,-fo|'e I'ictet f, !. I ide 11-i- ot i ! t o oht a i 11 1 1 \ < 1 1'o^el I 1 1 1 1 d e r ;J lea 1 | .1 e -> l| re
< The reaction li.-tw 'h, \ a n< I n peri MM 1 . •• I team i-adoiihleoue— tliat i-. thi-n
,,a\ IM lol'ined eilhel- i nl.oiii' o\lde.< (I a ccol'd ill- t o t 1 ie e. I lla t ioll I I ,< ) (' II . C'Ol, O]
THE COM POSITION OF WATER, HYDRO* i FA
The properties <>f ]ii/<Ii-<>>/i'n.— Hydrogen presents us with an example
of a gas which at first sight does not differ from air. It is not sur-
prising, therefore, that Paracelsus, having discovered that an aeriform
substance is obtained by the action of metals on sulphuric acid, did not
quite determine its difference from air. In fact, hydrogen, like air, is
colourless, and has no smell ; 21 but a more intimate acquaintance
with its properties proves it to be entirely different from air. The first
sign which distinguishes hydrogen from air is its combustibility. This
property is so easily observed that it is the one to which recourse is
usually had in order to recognise hydrogen, if it is evolved in a re-
action, although there are many other combustible gases. But before
speaking of the combustibility and other chemical properties of hydro-
gen, we will first describe the physical properties of this gas, as we did
in the case of water. It is easy to show that hydrogen is one of the
lightest gases.22 If passed into the bottom of a flask full of air,
carbonic anhydride C0.2 (according to the equation 2H..O + C = 2H.> + CO.2), and the result-
ing mixture is called water-gas ; we shall speak of it in describing the oxides of carbon.
-1 Hydrogen obtained by the action of zinc or iron on sulphuric acid generally smells
of hydrogen sulphide (like rotten eggs), which it contains in admixture. As a rule such
hydrogen is not so pure as that obtained by the action of an electric current or of sodium
on water. The impurity of the hydrogen depends on the impurities contained in the
zinc, or iron, and sulphuric acid, and on secondary reactions which take place simul-
taneously with the main reaction. Thus iron sulphide gives hydrogen sulphide
(FeS + HoSO4 = HoS + FeSO4). However, the hydrogen obtained in this manner may be
easily freed from the impurities it contains : some of them — namely those having acid
properties — are absorbed by caustic soda, and therefore may be removed by passing the
hydrogen through a solution of this substance ; another series of impurities is absorbed
by a solution of mercuric chloride ; and, lastly, a third series is absorbed by a solution of
potassium permanganate. The hydrogen may be dried by passing it over sulphuric acid
or calcium chloride. The substances serving for purifying the hydrogen are either
placed in Woulfe's bottles, or in tubes containing pumice stone moistened with the
purifying agent. The surface of contact is then greater, and the purification proceeds
more rapidly. If it be desired to procure completely pure hydrogen, it is sometimes
obtained by the decomposition of water (previously boiled to expel all air, and mixed
with pure sulphuric acid), by the galvanic current. Only the gas evolved at the negative
electrode is collected. Or else, an apparatus like that which gives detonating gas is used,
only the positive electrode being immersed under mercury containing zinc in solution.
The oxygen which is evolved at this electrode then immediately, at the moment of its
evolution, combines with the zinc, and this compound dissolves in the sulphuric acid and
forms zinc sulphate, which remains in solution, and therefore the hydrogen generated
will be quite free from oxygen.
'-'- An inverted beaker is attached to one arm of the beam of a rather sensitive
balance, and its weight counterpoised by weights in the pan attached to the other arm.
If the beaker be then filled with hydrogen it rises, owing to the air being replaced
by hydrogen. Thus, at the ordinary temperature of a room, a litre of air weighs
about 1'2 grams, and on replacing the air by hydrogen a decrease in weight of about 1
irrum per litre is obtained. Moist hydrogen is heavier than dry — for aqueous vapour
is nine times heavier than hydrogen. In filling balloons it is usually calculated that (it
being impossible to have perfectly dry hydrogen or to obtain it quite free from air)
the lifting force is equal to 1 kilogram ( = 1,000 grams) per cubic metre ( = 1,000 litres).
VOL. I. K
130
PRINCIPLES OF CHEMISTRY
hydrogen will not remain in it, but, owing to its lightness, rapidly
escapes and mixes with the atmosphere. If, however, a cylinder whoso
orifice is turned downwards be filled with hydrogen, it will not escape,
or, more correctly, it will only slowly mix with the atmosphere. This
may be demonstrated by the fact that a lighted taper sets fire to the
hydrogen at the orifice of the cylinder, and is itself extinguished inside
the cylinder. Hence hydrogen, being itself combustible, does not
support combustion. The great lightness of hydrogen is taken advan-
tage of for balloons. Ordinary coal gas, which is often also used for
the same purpose, is only about twice as light as air, whilst hydrogen is
•I 4^ times lighter than air. A very simple experiment with soap bubbles
very well illustrates the application of hydrogen for filling balloons.
Charles, of Paris, showed the lightness of hydrogen in this way, and con-
structed a balloon filled with hydrogen almost simultaneously with Mont-
golfier. One litre of hydrogen23 at 0° and 760 mm. pressure weighs
-3 The density of hydrogen in relation to the air has been determined by accurate
experiments. The first determination, made by Lavoisier, was not entirely exact ; taking
the density of air as unity, he obtained 0'0769 for that of hydrogen — that is, hydrogen as
thirteen times lighter than air. Later determinations have corrected this figure, the
most accurate determinations being due to Thomsen, who obtained the figure 0*0698 ;
Berzelius and Dulong, who obtained 0'0688 ; and Dumas and Bunseii. who obtained
0-06945. But the most exact determination of all is, without doubt, due to Regnault.
He took two spheres of considerable capacity, which cm en hied equal volumes of air
(thus avoiding the necessity of any correction for weighing them in air). Both spheres
were attached to the scale pans of a balance. One was sealed up, and the other first
weighed empty and then full of hydrogen. Thus, knowing the weight of the hydrogen
filling the sphere, and the capacity of the sphere, it was easy to rind the weight of a litre
of hydrogen ; and, knowing the weight of a litre of air at the same temperature and
pressure, it was easy to calculate the density of hydrogen. Regnault, by these experi-
ments, found the average density of hydrogen to be 0'06926 in relation to air, or including
the necessary corrections 0'06949.
In this book I shall always refer the densities of all gases to hydrogen, and not
to air ; therefore, for the sake of clearness, I will cite the weight of a litre of dry pure
hydrogen in grams at a temperature t° and under a pressure H (measured in millimetres
of mercury at 0°, in long. 45°). The weight of a litre of hydrogen
1
= 0-08958 x x __
760 1 -
gram.
I) -008(57*
For aeronauts it is very useful to know, besides this, the weight of the air at different
heights, and I therefore insert the adjoining table, constructed on the basis of Glaisher's
Pressure
760 m.m.
700 „
650 „
600 ,.
550
500
450
400
350
300
250
Temperature
15° C.
11-0°
7'6°
4'3°
+ 1-0°
- 2'4°
- 5-8°
- 9-1°
-12-5°
-15-9°
-19-2°
ICotatan
Height
\Vci-litc.f the :ur
60 p.c.
0 ni-tves 12'22 kilos.
64 „
<;<><> „ 1141 ,
64 „
1300 „
1073 ,
T.
£
63 „
1960 „
1003 ,
~%
(52 „
2660 „
',.:;! ,
58 „
3420 „
s.-,7 ,
-
52 .,
4250 „
7si :
"^
44 „
5170 703 ,
B
:!(•, ., 61!»0 „ i-,-24 ,
\
27 „ 7:-!i'>0 „ ."-I-2 ,
18 „ S7-20 „ 4.17 „ 1
THE n>M POSITION OF WATER, HYDROGEN 131
O08957S gram ; that is, hydrogen is almost 1-U (more exactly, 14-43)
times lighter than air. It is the lightest of all gases. The small density
of hydrogen determines many remarkable properties which it shows ;
thus, hydrogen flows exceedingly rapidly from fine orifices, its molecules
(Chap. I.) being endued with the greatest velocity of movement.24 At
pressures somewhat higher than the atmospheric pressure, all other
gases exhibit a greater compressibility and co-efficient of expansion than
they should according to the laws of Mariotte and Gay-Lussac ; whilst
hydrogen, on the contrary, is less compressed than should follow from
the law of Mariotte,'2"' and with a rise of pressure it expands slightly
data, for the temperature and moisture of the atmospheric strata in clear weather. All
the figures are given in the metrical system — 1000 millimetres = 39'37 inches, 1000 kilo-
grams = '220 l-:',:',7"> 11 >s., 1000 cubic metres = 35316'5 cubic feet. The starting temperature
at the earth's surface is taken as = 15° C., its moisture 60 p.c., pressure 760 millimetres.
The pressures are taken as indicated by an aneroid barometer, assumed to be corrected
at the sea level and at long. 45°.
Although the figures of this table are calculated with every possible care from average
data, yet they can only be taken for an elementary judgment of the matter, for in every
separate case the conditions, both at the earth's surface and in the atmosphere, will differ
from those here taken. In calculating the height to which a balloon can ascend, it is
evident that the density of gas in relation to air must be known.' This density for
ordinary coal gas is from 0'6 to 0'35, and for hydrogen with its ordinary contents of
moisture and air from O'l to 0'15.
Hence, for instance, it may be calculated that a balloon of 1000 cubic metres capacity
filled with pure hydrogen, and weighing (the envelope, tackle, people, and ballast) 727
kilograms, will ascend to a height of not much more than 4250 metres.
24 If a cracked flask be filled with hydrogen and its neck immersed under water or
mercury, then the liquid will rise up into the flask, owing to the hydrogen passing
through the cracks about 3'8 times quicker than the air is able to pass through these
cracks into the flask. The same thing may be better seen if, instead of a flask, a tube
whose end is closed by a porous substance, such as graphite, unglazed earthenware, or a
gypsum plate, be employed.
25 According to Boyle and Mariotte's law, for a given gas at a constant temperature the
volume decreases by as many times as the pressure increases; that is, this law requires
that the product of the volume v and the pressure p for a given gas should be a constant
quantity: pv — C, a constant quantity which does not vary with a change of pres-
sure. In reality this equation does very nearly and exactly express the observed rela-
tion between the volume and pressure, but only within comparatively small variations
of pressure, density, and volume. If these variations be in any degree considerable, the
quantity /tv proves to be dependent on the pressure, and it either increases or diminishes
with an increase of pressure. In the former case the compressibility is less than it
should be according to Mariotte's law, in the latter case it is greater. We will call the
.tii -4 case a positive discrepancy (because then d (pv) VZ (p) is greater than zero), and the
second case a negative discrepancy (because then d (pv) /d (p) is less than zero). Deter-
minations made by myself, M. L. Kirpicheff, and Hemilian showed that all known gases
at low pressures, when considerably rarefied, present positive discrepancies. On the
other hand it appears from the researches of Cailletet, Natterer, and Amagat that all
gases under great pressures (when the volume obtained is 500-1000 times less than
iimler the atmospheric pressure) also present positive discrepancies. Thus under a pres-
>nre of 2700 atmospheres air is compressed, not 2700 times, but only 800, and hydrogen
1000 times. Hence the positive kind of discrepancy is, so to say, normal to gases. And
this is easily understood. Did a gas follow Mariofcte's law, or were it compressed to a
K 2
132 PRINCIPLES OF CHEMISTRY
less than at the atmospheric pressure.26 However, hydrogen, like-
air and many other gases which are permanent at the ordinary tem-
greater extent than is shown by this law, then under great pressures it would attain a
density greater than that of solid and liquid substances, which is in itself improbable and
even impossible by reason of the fact that solid and liquid substances are themselves but
little compressible. For instance, a cubic centimetre of oxygen at 0D and under the at-
mospheric pressure weighs about 0-0014 gram, and at a pressure of 3000 atmospheres
(this pressure is attained in guns) it would, if it followed Mariotte's law, weigh 4'2 grams —
that is, would be about four times heavier than water — and at a pressure of 10000 atmo-
spheres it would be heavier than mercury. Besides this, positive discrepancies are pro-
bable in the sense that the molecules of a gas themselves must occupy a certain volume.
Admitting that Mariotte's law only applies to the intermolecular space still we find the
necessity of positive discrepancies. If we designate the volume of the molecules of a gas
by 6 (like Van der Waals, see Chap. I. note 34), then it must be expected that^> (v — b) = C.
Hence pv~C + bp, which expresses a positive discrepancy. Supposing that for hydrogen
j>y = 1000, at a pressure of one metre of mercury, according to the results of Regnault 's,
Amagat's, and Natterer's experiments, we obtain b as approximately 0'7 to 0*9.
Thus the increase of pv with the increase of pressure must be considered as the
normal law of the compressibility of gases. Hydrogen presents such a positive compres-
sibility at all pressures, for it presents positive discrepancies from Mariotte's law, accord-
ing to Regnault, at all pressures above the atmospheric pressure. Hence hydrogen is,
so to say, a sample gas. No other gas behaves so simply with a change of pressure. All
other gases at pressures from 1 to 30 atmospheres present negative discrepancies— that
is, they are then compressed to a greater degree than should follow from Mariotte's law,
as was shown by the determinations of Regnault, which were verified when repeated by^
myself andBoguzsky. Thus, for example, on changing the pressure from 4 to 20 metres
of mercury — that is, on increasing the pressure five times — the volume only decreased
4'93 times when hydrogen was taken, and 5'06 when air was taken.
The discrepancies from the law of Boyle and Mariotte for considerable pressures
(from 1 to 3000 atmospheres) are well expressed (for constant temperatures) by the
above-mentioned formula of Van der Waals (Chap. I. Note 34) ; Clausius' formula is more
closely approximate, but as it and Van der Waals' formula also do not in any way express
the existence of positive discrepancies from the law at low pressures, and as, accord-
ing to the above-mentioned determinations made by myself, Kirpicheff, and Hemilian and
verified (by two methods) by K. D. Kraevitch, they are proper to all gases (even to those
which are easily compressed into a liquid state, such as carbonic and sulphurous anhy-
drides) ; therefore these formulae, whilst accurately interpreting the phenomena of con-
densation and even of liquefaction, do not answer in the case of a high rarefaction of
gases — that is, to that instance where a gas approaches to a condition of maximum dis-
persion of its molecules, and perhaps presents a passage towards the substance termed
' luminiferous ether ' which fills up interplanetary and interstellar space. If we suppose
that gases are rarefiable to a definite limit only, having attained which they (like solids)
do not alter in volume with a decrease of pressure, then on the one hand the passage of
the atmosphere at its upper limits into a homogeneous ethereal medium becomes com-
prehensible, and on the other hand it would be expected that gases would, in a state of
high rarefaction (i.e., when small masses of gases occupy large volumes, or when furthest
removed from a liquid state) present positive discrepancies from Boyle and Mariotte's law.
Our present acquaintance with this province of highly rarefied gases is most limited, and
its further development promises to elucidate much in respect to natural phenomena. To-
the three states of matter (solid, liquid, and gaseous) it is evident a fourth must be yet
added, the ethereal or ultra-gaseous (as Crookes proposed), understanding by tins
matter in its highest possible state of rarefaction.
26 The law of Gay-Lussac states that all gases in all conditions present one coefficient
of expansion 0'00367 ; that is, when heated from 0° to 100° they expand like air;
namely, a thousand volumes of a gas measured at 0° will occupy 1367 volumes at 100°.
THE COMPOSITION OF WATER, HYDROGEN 133
perature, does not pass into a liquid state under a very consider-
able pressure,-7 but is compressed into a lesser volume than would
Regnault, about 1850, showed that Gay-Lussac's law is not entirely correct, and that
different gases, and also one and the same gas at different pressures, have not quite the
same coefficients of expansion. Thus the expansion of air between 0° and 100° is 0'367
under the ordinary pressure of one atmosphere, and at three atmospheres it is 0'371, the
expansion of hydrogen is 0'366, and of carbonic anhydride 0'37. Regnault, however, did
not directly determine the change of volume between the 0° and 100°, but measured the
variation of tension with the change of temperature ; but as gases do not entirely follow
Mariotte's law, therefore the change of volume cannot be directly judged by the variation
of tension. The investigations carried on by myself and Kayander, about 1870, showed
the direct variation of volume on heating from O3 to 100°. These investigations confirmed
Regnault's conclusion that Gay-Lussac's law is not entirely correct, and further showed
(1) that the expansion per volume from 0° to 100 J under a pressure of one atmosphere,
for air -0-368, for hydrogen = 0'367, for carbonic anhydride = 0'373, for hydrogen bromide
= 0'386, &c. ; (2) that for gases which are more compressible than should follow
from Mariotte's law the expansion by heat increases with the pressure — for example,
for air at a pressure of three and a half atmospheres, it equals 0'371, for carbonic
anhydride at one atmosphere it equals 0'373, at three atmospheres 0'389, and at eight
.atmospheres 0'413 ; (3) that for gases which are less compressible than should follow
from Mariotte's law, the expansion by heat decreases with an increase of pressure' —
for example, for hydrogen at one atmosphere 0'367, at eight atmospheres 0'369, for air at
a quarter atmosphere 0"370, at one atmosphere 0'368 ; and hydrogen like air (and all
gases) is less compressed at low pressures than should follow from Mariotte's law (air
at higher pressures than the atmospheric pressure gives a contrary result), as investiga-
tions made by myself, aided by Kirpicheff and Hemilian, showed. Hence, hydrogen,
starting from zero to the highest pressures, exhibits a gradually, although only slightly,
varying coefficient of expansion, whilst for air and other gases at the atmospheric and
higher pressures, the coefficient of expansion increases with the increase of pressure, so
long as their compressibility is greater than should follow from Mariotte's law. But
when at considerable pressures, this kind of discrepancy passes into the normal (see Note
25), then the coefficient of expansion of all gases decreases with an increase of pressure,
as is seen from the researches of Amagat. The difference between the two coefficients
of expansion, for a constant pressure and for a constant volume, is explained by these
relations. Thus, for example, for air at a pressure of one atmosphere the true coefficient
of expansion (the volume varying at constant pressure) = 0'00368 (according to Mende-
leeff and Kayander) and the variation of tension (at a constant volume, according to
Regnault) =0'00367.
27 Permanent gases are such as cannot be liquefied by an increase of pressure alone.
With a rise of temperature, all gases and vapours become permanent gases. As we shall
afterwards learn, carbonic anhydride becomes a permanent gas at temperatures above
31°, and at lower temperatures it has a maximum tension, and may be liquefied by
pressure alone.
The liquefaction of gases, accomplished by Faraday (see Ammonia) and others, in
the first half of this century, showed that a number of substances are capable, like water,
of taking all three physical states, and that' there is no essential difference between
vapours and gases, the only distinction being that the boiling points (or the temperature
at which the tension =760 mm.) of liquids lie above the ordinary temperature, and those
of liquefied gases below, and consequently a gas is a superheated vapour, or vapour
heated above the boiling point, or removed from saturation, rarefied, having a lower
tension than that maximum which is proper to a given temperature and substance. We
will here cite, as we did for water (p. 54), the maximum tensions of certain liquids and
gases at various temperatures, because they may be taken advantage of for obtaining
constant temperatures by changing the pressure at which boiling or the formation of
134 PRINCIPLES OF CHEMISTRY
follow from Marietta's law.28 From tins it may be concluded
that the absolute boiling point of hydrogen, and of gases resembling
saturated vapours takes place. The temperatures (according to the air thermometer)
are placed on the left, and the tension in millimetres of mercury (at 0 ) on the right,
hand side of the equations. Carbon bisulphide, CSg, 0° = 127'9; 10° = 198'5; 20° = 298*1;
30° = 431-6; 40° = 617'5; 50° = 857'1. Chlorobenzene, CCH5C1, 70° = 97'9; 80° = l-il'8;
90° = 208'4; 100° = 292'8; 110° = 402-6; 120° = 54t2'.s; 13u- = 71iK). Aniline, C6H7N,
150° = 283-7; 160° = 887-0; 170° = 515-6; 180° = 677"2; LS.V =771-5. Methyl sulicylate,
C8H8O5, 180° = 249-4; 1900 = 330'9; 200° = 432'4; 210C = 557'5; 220C = 710"2; 224': =77i»".».
Mercury, Hg, 300° = 246'8 ; 310° = 304'9 ; 3200 = 373'7 ; 3300 = 454'4; 340° = 548-6;
350° = 658'0; 359° = 770'9. Sulphur, S, 395° = 300; 423° = 500; 443° =-700 ; 452° = 800 ;
459° = 900. These figures (Ramsay and Young) show the possibility of fixing con-
stant temperatures in the vapours of boiling liquids. The tension of liquefied
gases is expressed in atmospheres. Sulphurous anhydride, SO>£ — bOc = 0'4 ; —20° = 0*6;
-10° = 1; 0° = 1'5; +10° = 2'3; 20° = 3'2; 30°=5'3. Ammonia, NH5, -40° = 0'7;
-30° = 1-1; -20° = l-8; -10° = 2'8; 0° = 4'2; + 10° = 6'0; 20° = 8'4. Carbonic anhydride,
CO2, -115° = 0-033 ; -80° = 1; — 70° = 2'1; -600 = 3'9 ; -50° = 6'8 ; -40° = 10; -20° = 23;
0° = 35; +10° = 46; 20°=58. Nitrous oxide, N2O, -125° = 0'033 ; -92° = 1; -80° = 1'9;
-50° = 7-6; -20° = 23-1; 00 = 36'1; +20° = 55'3. Ethylene, CoH4, -140° = 0'033;
-130° = 0-1; -103° = 1; -40° = 13; -1° = 42. Air, -191° = 1; -15~8° = 14; -140° = 39.
Nitrogen, N2, -203° = 0'085; -193° = 1; -160° = 14; -146° = 32. The methods of
liquefying gases (by pressure and cold) will be described under ammonia, nitrous oxide,
sulphurous anhydride, and in later footnotes. We will now turn our attention to the
fact that the evaporation of volatile liquids, under various, and especially under lowr
pressures, gives an easy means for obtaining low temperatures. Thus liquefied carbonic
anhydride, under the ordinary pressure, reduces the temperature to - 803, and when it
evaporates in an atmosphere rarefied (in an air-pump) to 25 mm. ( = 0'033 atmospheres)
the temperature, judging by the above-cited figures, falls to -115° (Dewar). Even the
evaporation of liquids of common occurrence, under low pressures easily attainable in an
air-pump, may produce low temperatures, which may be again taken advantage of for ob-
taining still lower temperatures. Water boiling in a vacuum becomes cold, and under
a pressure of less than 4'5 mm. it freezes, because its tension at 0° is 4'5 mm. A
sufficiently low temperature may be obtained by forcing fine streams of air through
common ether, or liquid carbon bisulphide, CS2, or methyl chloride, CH3C1, and other
similar volatile liquids. In the adjoining table are given, for certain gases, (1) the
number of atmospheres necessary for their liquefaction at 15°, and (2) the boiling points
of the resultant liquids under a pressure of 760 mm.
C2H4 NaO CO., H.,S AsH3 NH*3 HC1 CH3C1 CLX, SO,
(1) 42 31 52 10 8 7 25 4 4 3
(2) -103° -92° -80° -74° -58° -38° -35° -24° -21° -10°
28 Natterer's determinations (1851-1854), together with Amagat's results (1880-1888),
show that the compressibility of hydrogen, under high pressures, may be expressed by
the following figures : —
p. 1 100 1000 2500
v I 0-0107 0-0019 0-0013
2W 1 1-07 1-9 3-25
s 0-11 10-3 58 85
where p = the pressure in metres of mercury, v = the volume, if the volume taken under
a pressure of 1 metre =1, and s the weight of a litre of hydrogen at 20° in grams. If
hydrogen followed Mariotte's law, then under a pressure of 2500 metres, one litre would
contain not 85, but 265, grams. It is evident from the above figures that the weight of
a litre of the gas approaches a limit as the pressure increases, which is doubtless the
density of the gas when liquefied, and therefore the weight of a litre of liquid
hydrogen will probably be near 100 grams (density about O'l, being less than that of all
other liquids).
THE COMPOSITION OF WATER, HYDROGEN 135
it,2<J lies very much below the ordinary temperature ; that is, that the
liquefaction of this yas is only possible at low temperatures, and under
2) Cagniard de Latour, on heating ether in a closed tube to about 190°, observed that
at this temperature the liquid is transformed into vapour occupying the original volume
— that is, having the same density as the liquid. The further investigations made by
Drion and myself, showed that every liquid has such an absolute boiling point, above which
it cannot exist as a liquid and is transformed into a dense gas. In order to grasp the true
signification of this absolute boiling temperature, it must be remembered that the liquid
state is characterised by a cohesion of its particles which does not exist in vapours and
gases. The cohesion of liquids is expressed in their capillary phenomena (the breaks
in a column of liquid, drop formation, and rise in capillary tubes, &c.), and the product of
the density of a liquid into the height to which it rises in a capillary tube (of a definite
diameter) may serve as the measure of the magnitude of cohesion. Thus, in a tube of
2 mm. diameter, water at 15° rises (the height being corrected for the meniscus) 14'8mm.,
and ether at t° to a height 5'35 — 0'028 tc mm. The cohesion of a liquid is lessened by
heating, and therefore the capillary heights are also diminished. It has been shown
by experiment that this decrement is proportional to the temperature, and hence by the
aid of capillary observations we are able to form an idea that at a certain rise of
temperature the cohesion may become = 0. For ether, according to the above formula,
this would happen at 191°. If the cohesion disappear from a liquid it becomes a gas,
for cohesion is the only point of difference between these two states. A liquid in
evaporating and overcoming the force of cohesion absorbs heat. Therefore, the absolute
boiling point was defined by me (1861) as that temperature at which (a) a liquid cannot
exist as a liquid, but forms a gas which cannot pass into a liquid state under any
pressure whatever ; (b) cohesion = 0; and (c) the latent heat of evaporation = 0.
These ideas were but little spread until Andrews (1869) explained the matter from
another aspect. Starting from gases, he discovered that carbonic anhydride can-
not be liqnefied by any degree of compression at temperatures above 81°, whilst at
lower temperatures it can be liquefied. He called this temperature the critical tem-
perature. It is evident that it is the same as the absolute boiling point. We shall after-
wards designate it by tc. At low temperatures a gas which is subjected to a pressure
greater than its maximum tension (Note 27) is completely transformed into a liquid,
which, in evaporating, gives a saturated vapour which possesses this maximum tension ;
whilst at temperatures above tc the pressure to which the gas is subjected may increase
indefinitely. However, under these conditions the volume of the gas does not change
indefinitely but approaches a definite limit (see Note 28) — that is, it resembles in this
respect a liquid or a solid which is altered but little in volume by pressure. The
volume which a liquid or gas occupies at tc is termed the critical volume, which corre-
sponds with the critical pressure, which we will designate byjpc and express in atmo-
spheres. It is evident from what has been said that the discrepancies from Mariotte
and Boyle's law, the absolute boiling point, the density in liquid and compressed
gaseous states, and the properties of liquids, must all be intimately connected together.
We will consider these relations in one of the following notes. At present we will
supplement the above observations by the values of tc and pc for certain liquids and
gases which have been investigated in this respect —
„
p.c.
t.c. p.c.
N2 - 14f,
88
H2S + 108°
92
CO - 140°
39
C2N2 + 124° 62
O.2 - 119°
50
NH3 + 181° 114
CH4 - 100°
50
CH3C1 + 141°
78
NO - 93°
CsHt + 10°
71
51
SO2 + 155°
C5H10 + 192°
79
84
CO* + 82°
77 C4H10O + 193°
40
N20 + 53°
75
CHC13 + 268°
55
C2Ho + 87°
68
CS., + 278°
78
HCf H 52°
86
C6H6 + 292°
60 .
136
PRINCIPLES OF CHEMISTRY
great pressures.30 This conclusion was verified (1879) by tire ex-
periments of Pictet and Cailletet.31 They compressed gases at a
30 This conclusion was arrived at by me in 1870 (Ann. Phys. Chem. 141, 023).
31 Pictet, in his researches, effected the direct liquefaction of many gases which up to
that time had not been liquefied. He employed the apparatus used for the manufacture
of ice on a large scale, employing the vaporisation of liquid sulphurous anhydride
which may be liquefied by pressure alone. This anhydride is a gas which is transformed
into a liquid at the ordinary temperature under a pressure of several atmosphere-
Note 27), and boils at —10° at the ordinary atmospheric pressure. This liquid, like all
others, boils at a lower temperature under a diminished pressure, and by continually
pumping out the gas which comes off by means of a powerful air-pump its boiling point
falls as low as —75°. Consequently, if we on the one hand force liquid sulphurous
anhydride into a vessel, and on the other hand pump out the gas from the same vessel
by powerful air-pumps, then the liquefied gas will boil in the vessel, and cause the tempera-
ture in it to fall to — 7S3. If a second vessel is placed inside this vessel, then another
gas may be easily liquefied in it at the low temperature produced by the boiling liquid
sulphurous anhydride. Pictet in this manner easily liquefied carbonic anhydride, COo
(at —60° under a pressure of from four to six atmospheres). This gas is more refractory
to liquefaction than sulphurous anhydride, but for this reason it gives on evaporating a
still lower temperature than can be attained by the evaporation of sulphurous anhydride.
A temperature of — 80° may be obtained by the evaporation of liquid carbonic anhydride at
a pressure of 760 mm., and in an atmosphere rarefied by a powerful pump the temperature
falls to —140°. By employing such low temperatures, it was possible, with the aid of
.pressure, to liquefy the majority of the other gases. It is evident that special pumps
which are capable of rarefying gases are necessary to reduce the pressure in the
chambers in which the sulphurous and carbonic anhydride boil ; and that, in order to
re-condense the resultant gases into liquids, special force pumps are required for pumping
the liquid anhydrides into the refrigerating chamber. Thus, in Pictet's apparatus
(fig. 24), the carbonic anhydride was liquefied by the aid of the pumps E F, which com-
FIG. 24. — General arrangement of the apparatus employed by Pictet for liquefying gases.
THE COMPOSITION OK WATKI;. HYDROGEN 137
very low temperature, and then allowed them to expand, either by
directly decreasing the pressure or by allowing them to escape into the
air, by which means the temperature fell still lower, and then, just as
steam when rapidly rarefied3- deposits liquid water in the form of a
pressed the gas (;lt il pressure of 4-6 atmospheres) and forced it into the tube K,
vigorously cooled by being surrounded by boiling liquid sulphurous anhydride, which
was condensed in the tube C by the pump B, and rarefied by the pump A. The
liquefied carbonic anhydride flowed down the tube K into the tube H, in which it was
subjected to a low pressure by the pump E, and thus gave a very low temperature of
about 140°. The pump E carried off the vapour of the carbonic anhydride, and conducted it
to the pump F, by which it was again liquefied. The carbonic anhydride thus made an
entire circuit — that is, it passed from a rarefied vapour of small tension and low tempera-
ture into a compressed and cooled gas, which was transformed into a liquid, which
again vaporised and produced a low temperature.
Inside the wide inclined tube H, where the carbonic acid evaporated, was placed a
second and narrow tube M containing hydrogen, which was evolved in the vessels L
from a mixture of sodium formate and caustic soda (CHOoNa + NaHO^Na^COs + Ho).
This mixture gives hydrogen on heating the vessel L. This vessel and the tube M were
made of thick copper, and could withstand great pressures. They were, besides, her-
metically connected together and closed up. Thus the hydrogen which was evolved had
no outlet, accumulated in a limited space, and its pressure increased in proportion to
the amount of it evolved. The magnitude of this pressure was recorded on a metallic
manometer E attached to the end of the tube M. As the hydrogen in this tube was sub-
mitted to a very low temperature and a powerful pressure, there were all the necessary con-
ditions for its liquefaction. When the pressure in the tube H became steady — i.e., when
the temperature had fallen to — 140 J, and the manometer R indicated a pressure of 650
atmospheres in the tube M — then this pressure did not rise with a further evolution of
hydrogen in the vessel L. This served as an indication that the tension of the vapour of
the hydrogen had attained a maximum corresponding with —140°, and that consequently
all the excess of the gas was condensed to a liquid. Pictet convinced himself of this
by opening the cock N, when the liquid hydrogen rushed out from the orifice. But, on
leaving a space where the pressure was equal to 650 atmospheres, and coming into contact
with air under the ordinary pressure, the liquid or powerfully-compressed hydrogen
expanded, began to boil, absorbed still more heat, and became still colder. In doing so
a portion of the liquid hydrogen, according to Pictet, passed into a solid state, and did
not fall in drops into a vessel placed under the outlet N, but as pieces of solid matter,
which struck against the sides of the vessel like shot and immediately vaporised.
Thus, although it was impossible to see and keep the liquefied hydrogen, still it was
admitted that it passed not only into a liquid, but also into a solid, state, because Pictet
in his experiments obtained other gases which had not previously been liquefied,
especially oxygen and nitrogen, in a liquid and solid state. Pictet supposed that liquid
and solid hydrogen have the properties of a metal, like iron.
3- At the same time (1879) as Pictet was working on the liquefaction of gases in
Switzerland, Cailletet, in Paris, was occupied en the same subject, and his results,
air hough not so convincing as Pictet's, still showed that the majority of gases, previously
unliquefied, were capable of passing into a liquid state. Cailletet subjected gases to a
pressure of several hundred atmospheres in thin glass tubes (fig. 25) ; he then cooled
the compressed gas as far as possible by surrounding it with a freezing mixture; a
•cock was then rapidly opened for the outlet of mercury from the tube containing the gas,
which consequently rapidly and vigorously expanded. This rapid expansion of the gas
would produce great cold, just as the rapid compression of a gas evolves heat and causes
a rise in temperature. This cold was produced at the expense of the gas itself, for in
rapidly expanding its particles were not able to absorb heat from the walls of the
tube, and in cooling a portion of the expanding gas was transformed into liquid. This
138
PRINCIPLES OF CHEMISTKY
fog, hydrogen in expanding forms a fog, thus indicating its passage into
a liquid state. But as yet it has been impossible to preserve this
liquid, even for a short time, to determine its properties, notwithstanding
the employment of a temperature of — 200° and a pressure of 200 atmo-
spheres,33 although by these means the gases of the atmosphere may be
kept in a liquid state for a long time. This is naturally dependent
on the fact that the absolute boiling point of hydrogen lies lower than
that of all other known gases, which is related to the extreme lightness
of hydrogen.34
was seen from the formation of cloud-like drops, like a fog, which rendered the gas opaque.
Thus Cailletet proved the possibility of the liquefaction of gases, but lie did not isolate
the liquids. The method of Cailletet allows the passage of
gases into liquids being observed with greater facility and
simplicity than Pictet's method, which requires a very
complicated and expensive apparatus.
The methods of Pictet and Cailletet were afterwards
improved by Olszewski, Wroblewski, Dewar, and others.
In order to obtain a still lower temperature they employed
liquid ethylene or nitrogen instead of carbonic acid gas,
whose evaporation at low pressures produces a much lower
temperature (to —200°). They also improved on the
methods of determining such low temperatures, but the
methods were not essentially altered ; they obtained nitro-
gen and oxygen in a liquid, and nitrogen even in a solid,
state, but no one has yet succeeded in seeing hydrogen in
a liquid form.
55 The investigations of C. Wroblewski in Cracow
clearly proved that Pictet could not have obtained liquid
hydrogen in the interior of his apparatus, and that if he
did obtain it, it could only have been at the moment of its
outrush due to the fall in temperature following its sud-
den expansion. Pictet calculated that he obtained a tem-
perature of — 140°, but in reality it hardly fell below — 120°,
judging from the latest data for the vaporisation of car-
bonic anhydride under low pressure. The diffei*ence lies
in the method of determining low temperatures. Judging
from other properties of hydrogen (see Note 34), one would
think that its absolute boiling point lies far below - 120°,
and even ~140° (according to the calculation of Sarrau, on
the basis of its compressibility, at - 174°). But even at -200°
(if the methods of determining such low temperatures be correct) hydrogen does not give
a liquid even under a pressure of several hundred atmospheres. However, on expan-
sion a fog is formed and a liquid state attained, but the liquid does not separate.
54 After the conception of the absolute temperature of ebullition (tc, note 211) had
been worked out (about 1870), and its connection with the deviations from Mariotte's law
had become evident, and especially after the liquefaction of permanent gases, general
attention was turned to the development of the fundamental conceptions of the gaseous
and liquid states of matter. Some investigators directed their energies to the further
study of vapours (for instance, Ramsay and Young), gases (for instance, Amagat), and
liquids (for instance, Zaencheffsky, Nadeschdin, and others), especially to liquids near tc
and pc ; others (for instance, Konovaloff and De Haen) endeavoured to discover the rela-
tion between liquids under ordinary conditions (removed from tc and pc) and gases,
forli°uef In*'* ases
THE COMPOSITION OF WATER, HYDK< Kil-N IB'J
Although a substance which passes with great difficulty into a
liquid state by the action of physico-mechanical forces, hydrogen loses
while a third class of investigators (Van der Waals, Clausius, and others ), starting from the
already generally-accepted principles of the mechanical theory of heat and the kinetic
theory of gases, and having made the self-evident proposition of the existence in jj
of those forces which clearly act in liquids, deduced the connection between the properties
of one and the other. It would be out of place in an elementary handbook like the
present to enunciate the whole mass of conclusions arrived at by this method, but it is
necessary to give an idea of the results of Van der Waals' considerations, for they explain
the gradual uninterrupted passage from a liquid into a gaseous state in the simplest
form, and, although the deduction cannot be considered as complete and decisive (see
note 25), nevertheless it penetrates so deeply into the essence of the matter that its
signification is not only reflected in a great number of physical investigations, but also in
the province of chemistry, where instances of the passage of substances from a gaseous
to a liquid state are so common, and where the very processes of dissociation, decomposi-
tion, and combination must be identified with a change of physical state of the partici-
pating substances.
For a given quantity (weight, mass) of a definite substance, its state is expressed
by three variables— volume v, pressure (elasticity, tension) p, and temperature t.
Although the compressibility — [i.e., d(v)d(p)] — of liquids is small, still it is clearly ex-
pressed, and varies not only with the nature of liquids but also with their pressure and
temperature (at tc the compressibility of liquids is very considerable). Although gases,
according to Mariotte's law, with small variations of pressure, are uniformly compressed,
nevertheless the dependence of their volume v on t and p is very complex. The same
applies to the coefficient of expansion [ = d(v)d(t), or d(p)d(t)], which also varies with
t and_p, both for gases (see Note 26), and for liquids (at tc it is very considerable, and
often exceeds that of gases, 0'00367). Hence the equation of state must include three
variables — v, p, and t. For a so-called perfect (ideal) gas, or for inconsiderable variation
of density, the elementary expression pv = Ra(t + at), or pv — R (273 + 2) should be
accepted, where R is a constant varying with the mass and nature of a gas, as expressing
this dependence, because it includes in itself the laws of Gay-Lussac and Mariotte, for at
a constant pressure the volume varies proportionally to 1 + at, and when t is constant
the product of tv is constant. In its simplest form the equation may be expressed thus :
where T denotes what is termed the absolute temperature, or the ordinary temperature
+ 273- that is, T-2 + 273.
Starting from the supposition of the existence of an attraction or internal pressure
(expressed by a) proportional to the square of the density (or inversely proportional to
the square of the volume), and of the existence of a volume or length of path (expressed
by b) of gaseous molecules, Van der Waals gives for gases the following more complex
equation of state : —
(p+ a } (v -6) = 1+0-003672 ;
V 9* J
if at 0° under a pressure ^ = 1 (for instance, under the atmospheric pressure), the volume
(for instance, a litre) of a gas or vapour be taken as 1, and therefore v and b be expressed
by the same units as p and a. The deviations from both the laws of Mariotte and Gay-
Lussac are expressed by the above equation. Thus, for hydrogen a must be taken as
infinitely small, and 6 = 0'0009, judging by the data for 1000 and 2500 metres pressure
(Note 28). For other permanent gases, for which (Note 28) I showed (about 1870) from
Regnault's and Natterer's data, a decrement of pv, followed by an increment, which was .
confirmed (about 1880) by fresh determinations made by Amagat, this phenomena may
be expressed in definite magnitudes of a and b (although Van der Waals' formula is not
applicable for minimum pressures) with sufficient accuracy for contemporary require-
ments. It is evident that Van der Waals' formula can also express the difference of the
140 PRINCIPLES OF CHEMISTRY
its gaseous state (that is, its elasticity, or the physical energy of its
molecules, or their rapid progressive movement) with comparative ease
coefficients of expansion of gases with a change of pressure, and according to the
methods of determination (Note 26). Besides this, Van der Waals' formula shows that
at temperatures above 273 ( a — 1\ only one actual volume (gaseous) is possible,
whilst at lower temperatures, by varying the pressure, three different volumes— liquid,
gaseous, and partly liquid partly saturated-vaporous — are possible. It is evident that
the above temperature is the absolute boiling point — that is, (tc) = 273 f ~ — 1 J . It is
found under the condition that all three possible volumes (the three roots of Van der
Waals' cubic equation) are then similar and equal (vc = Sb). The pressure in this case
(we) = a 9. These ratios between the constants a and b and the conditions of critical
276
state — i.e. (tc) and (pc) — give the possibility of determining the one magnitude from the
other. Thus for ether (Note 29), (tc}= 193°, (*p) = 40, from whence a = 0'0307, 6 = 0'00533.
From whence (t>c) = 0'016. That mass of ether which at a pressure of one atmosphere at
0° occupies one volume — for instance, a litre — occupies, according to the above- mentioned
condition, this critical volume. And as the density of the vapour of ether compared with
hydrogen = 37, and a litre of hydrogen at 0° and under the atmospheric pressure weighs
0-089(5 grams, then a litre of ether vapour weighs 3'32 grams ; therefore, in a critical
state (at 193° and 40 atmospheres), 3'32 grams occupy 0*016 litres, or 16 c.c. ; therefore 1
"gram occupies a volume of about 5 c.c., and the weight of 1 c.c. of ether will then be 0'21.
According to the investigations of Kamsay and Young (1887), the critical volume of ether
was approximately such at about the absolute boiling point, but the compressibility of
the liquid is so great that the slightest change of pressure or temperature acts consider-
ably on the volume. ^But the investigations of the above savants gave another indirect
demonstration of the true composition of Van der Waals' equation. They also found for
ether that the isochords, or the lines of equal volumes, are generally straight lines if the
temperatures and pressures vary. For instance, the volume of 10 c.c. for 1 gram of ether
corresponds with pressures (expressed in metres of mercury) equal to 0'185£ — 8'3 (for
instance, at 180° and 21 metres pressure, at 280° and 34'5 metres pressure). The recti-
linear form of the isochord (then v — & constant quantity) is a direct result of Van der
Waals' formula.
When, in 1883, I demonstrated that the specific gravity of liquids decreases in propor-
tion to the rise of temperature [S, = S0-K£ or S,= S0 (1-Kf)], or that the volumes
increase in inverse proportion to the binomial 1 — K£, that is, V/ = V0 (1 — Ktf)"1, where K
is the modulus of expansion, which varies with the nature of the liquid (an exactitude of
the same kind as that by which for gases the volumes increase proportionately to the
binomial l + at), then, in general, not only does a connection arise between gases and
liquids with respect to a change of volume, but also it would appear possible, by availing
oneself of Van der Waals' formula, to judge, from the phenomena of the expansion of
liquids, as to their transition into vapour, and to connect together all the principal pro-
perties of liquids, which up to this time had not been considered to be in direct dependence.
Thus Thorpe and Riicker found that 2(f c) + 278 = 1/K, where K is the modulus of expan-
sion in the above-mentioned formula. For example, the expansion of ether is expressed
with sufficient accuracy from 0° to 100° by the equation S< = 0'786 (1-0'00154£), or V<
= 1 (1 — 0'00154£), where 0'00154 is the modulus of expansion, and therefore (tc) = lS8°, or
by direct observation 193°. For silicon tetrachloride, SiCl4, the modulus equals 0'00186,
. from whence (£c) = 231°, and by experiment 280°. On the other hand, D. P. Konovoloff,
admitting that the external pressure p in liquids is insignificant when compared with the
internal (a in Van der Waals' formula), and that the work in the expansion of liquids is
proportional to their temperature (as in gases), directly deduced, from Van der Waals'
formula, the above-mentioned formula for the expansion of liquids, Vt=-l/ (1 — Kt), and
TIIK COMPOSITION OF WATKK. HYDROGEN 141
under the influence of chemical attraction,3"' which is not only shown
from the fact that hydrogen and oxygen (two permanent gases) form
liquid water, but also from many phenomena of the absorption of
hydrogen.
Hydrogen is vigorously condensed by certain solids ; for example,
by charcoal and by spongy platinum. If apiece of freshly-ignited char-
coal be introduced into a cylinder full of hydrogen standing in a
mercury bath, then the charcoal absorbs as much as twice its volume
of hydrogen Spongy platinum condenses still more hydrogen. But
l><illadium, a grey metal which occurs with platinum, absorbs more
hydrogen than any other metal. Graham showed that when heated to
a red heat and cooled in an atmosphere of hydrogen, palladium retains
as much as 600 volumes of hydrogen. When once absorbed it retains
the hydrogen at the ordinary temperature, and only parts with it when
heated to a red heat.30 This capacity of certain dense metals for the
absorption of hydrogen explains the property of hydrogen of passing
through metallic tubes.37 It is termed occlusion, and presents a
also the magnitude of the latent heat of evaporation, cohesion, and compressibility under
pressure. In this way Van der Waals' formula embraces the gaseous, critical, and liquid
states of substances, and shows the connection between them. On this account, although
Van der Waals' formula cannot be considered as perfectly general and accurate, yet it is
not only very much more exact i\i&npv = RT but is also more comprehensive, because
it applies to both gases and liquids. Further research will naturally give further prox-
imity to truth, and will show the connection between composition and the constants
(a and b) ; but a great scientific progress is seen in this form of the equation of
state.
Clausius (in 1880), taking into consideration the variability of a, in Van der Waals'
formula, with the temperature, gave the following equation of state : —
Sarrau applied this formula to Amagat's data for hydrogen, and found a = 0'0551,
c = — 0-00043, b = G'00089, and therefore calculated its absolute boiling point as — 174°, and
(pc] = 99 atmospheres. But as similar calculations for oxygen ( — 105°), nitrogen ( — 124°),
and marsh gas ( — 76°) gave t c higher than it really is, therefore the absolute boiling point
of hydrogen must lie below — 174°.
55 This and a number of similar cases clearly show how great are the internal
chemical forces compared with physical and mechanical forces.
36 The capacity of palladium to absorb hydrogen, and in so doing to increase in
volume, may be easily demonstrated by taking a sheet of palladium varnished on one
side, and using it as a cathode. The hydrogen which is evolved by the action of the
current is retained by the unvarnished surface, as a consequence of which the sheet curls
up. By attaching a pointer (for instance, a quill) to the end of the sheet this bending
effect is rendered strikingly evident, and on reversing the current (when oxygen will be
evolved and combine with the absorbed hydrogen, forming water) it may be shown that
on losing the hydrogen the palladium regains its original fo'rm.
37 Deville discovered that iron and platinum become pervious to hydrogen at a red
heat. He speaks of this in the following terms : — ' The permeability of such homogeneous
substances as platinum and iron is quite different from the passage of gases through
such non-compact substances as clay and graphite. The permeability of metals depends
142 PRINCIPLES OF CHEMISTRY
similar phenomenon to solution ; it is based on the capacity of metals
of forming unstable easily dissociating compounds38 with hydrogen
similar to those which salts form with water.
At the ordinary temperature hydrogen very feebly and rarely enters
into chemical reaction. The capacity of gaseous hydrogen for reaction
becomes evident only under a change of circumstances — by compression,
heating, or the action of light, or at the moment of its evolution. How-
ever, under these circumstances it combines directly with only a very
few of the elements. Hydrogen combines directly with oxygen, sulphur,
carbon, potassium, and certain other elements, but it does not combine
directly with either the majority of the metals or with nitrogen, phos-
phorus, ifcc. Compounds of hydrogen with certain elements on which
it does not act directly are, however, known ; they are not obtained by
a direct method, but by reactions of decomposition, or of double decom-
position, of other hydrogen compounds. The property of Irj'drogen of
combining with oxygen at a red heat determines its combustibility.
We have already seen that hydrogen easily takes fire, and that it then
- on their expansion, brought about by heat, and proves that metals and alloys have a
certain porosity.' However, Graham proved that it is only hydrogen which is capable of
passing through the above-named metals in this manner. Oxygen, nitrogen, ammonia,
and many other gases, only permeate through in extremely minute quantities. Graham
showed that at a red heat about 500 c.c. of hydrogen pass per minute through a surface
of one square metre of platinum I'l mm. thick, but that with other gasea the amount
transmitted is hardly perceptible. Indiarubber has the same capacity for allowing the
transference of hydrogen through its substance (see Chap. III.), but at the ordinary tem-
perature one square metre, 0'014 mm. thick, transmits only 127 c.c. of hydrogen per
' minute. In the experiment on the decomposition of water by heat in porous tubes, the
clay tube may be exchanged for a platinum one with advantage. Graham showed that
by placing a platinum tube containing hydrogen under these conditions, and surrounding
it by a tube containing air, the transference of the hydrogen may be observed by the
decrease of pressure in the platinum tube. In one hour almost all the hydrogen (97 p.c.)
had passed from the tube, without being replaced by air. It is evident that the occlusion
and passage of hydrogen through metals capable of occluding it are not only intimately
connected together, but are dependent on the capacity of metals to form compounds of
various degrees of stability with hydrogen — like salts with water.
58 Palladium, as it appeared on further investigation, gives a definite compound,
PdoH (see further) with hydrogen ; but what was most instructive was the investigation
of sodium hydride, Na.2H, which clearly showed that the origin and properties of such
compounds are in entire accordance with the conceptions of dissociation. In the chapter
devoted to sodium we shall therefore speak more fully of this substance.
Being a gas which is difficult to condense, hydrogen is little soluble in water ami
other liquids. At 0° a hundred volumes of water dissolve 1'9 volumes of hydrogen, and
alcohol 6'9 volumes measured at 0° and 760 mm. Molten iron absorbs hydrogen, but in
solidifying, it expels it. The solution of hydrogen by metals is to a certain degree
based on its affinity for metals, and must be likened to the solution of metiils in mercury
and to the formation of alloys. In its chemical properties hydrogen, as we shall see
later, has much of a metallic character. Pictet (see Note 81) even affirms that liquid
hydrogen has metallic properties. The metallic properties of hydrogen are also evinced
in the fact that it is a good conductor of heat, which is not the case with other gases
(Magnus).
THE COMPOSITION OF AVATKR. II VI'IK >< , KN 143
burns with a pale — that is, non-luminous — flame.39 Hydrogen does not
combine with the oxygon of the atmosphere at the ordinary tempe-
rature ; but this combination takes place at a red heat,40 and is accom-
panied by the evolution of much heat. The product of this combination
is \vater — that is, a compound of oxygen and hydrogen. This is the
xy/^/^.v/'x i>f water, and we have already noticed its analysis or decom-
position into its component parts. The synthesis of water may be very
easily observed if a cold glass bell jar be placed over a burning hydrogen
Ha me, and, better still, if the hydrogen flame be lighted in the tube of
a condenser. The water will condense in drops as it is formed on the
walls of the condenser and trickle down.41
Light does not aid the combination of hydrogen and oxygen, so
that a mixture of these two gases does not change when exposed to the
action of light ; but an electric spark acts just like a flame, and this is
taken advantage of for inflaming a mixture of oxygen and hydrogen, or
detonating gas, inside a vessel, as will be explained in the following
chapters. As hydrogen (and oxygen also) is condensed by spongy
platinum, by which a rise of temperature ensues, and as platinum acts
by contact (p. 38), therefore hydrogen also combines with oxygen,
under the influence of platinum, as Dobereiner showed. If spongy
platinum be thrown into a mixture of hydrogen and oxygen, an explo-
sion takes place. If a mixture of the gases be passed over spongy
platinum, combination also ensues, and the platinum becomes red-hot.42
50 If it be desired to obtain a perfectly colourless hydrogen flame, it must issue from
a platinum nozzle, as the glass end of a gas-conducting tube imparts a yellow tint to the
Hume, owing to the presence of sodium in the glass.
40 Let us imagine that a stream of hydrogen passes along a tube, and let us mentally
divide this stream into several parts, consecutively passing out from the orifice of the
tube. The first part is lighted — that is, brought to a state of incandescence, in which
state it combines with the oxygen of the atmosphere. A considerable amount of heat is
e\ -nlved in the combination. The heat evolved then, so to say, ignites the second part of
hydrogen coming from the tube, and, therefore, when once ignited, the hydrogen con-
tinues to burn, if there be a continual supply of it, and if the atmosphere in which it
l)n rns be unlimited and contains oxygen.
41 The combustibility of hydrogen may be shown by the direct decomposition of water
by sodium. If a pellet of sodium be thrown into a cup containing water, then it floats
on the water and evolves hydrogen, which may be lighted. The presence of sodium imparts
;i yellow tint to the flame. If potassium be taken, the hydrogen bursts into flame of
itself, because sufficient heat is evolved in the reaction for the ignition and inflammation
of the hydrogen. The flame is rendered violet by the potassium. If sodium be thrown
not on water, but on an acid, it will evolve more heat, and the hydrogen will then also
burst into flame. These experiments must be carried on with caution, as sometimes
towards the end a mass of sodium oxide (Note 8) is produced, and flies about; therefore
it is best to cover the vessel in which the experiment is carried on.
'- This property of spongy platinum is made use of in the so-called hydrogen cigar-
light. It consists of a glass cylinder or beaker, inside which there is a small lead stand
i which is not acted on by sulphuric acid), on which a piece of zinc is laid. This zinc is
covered by a bell, which is open at the bottom and furnished with a cock at the top.
144 PRINCIPLES OF CHEMISTRY
Although gaseous hydrogen does not act directly43 on many sub-
stances, yet in a nascent state reaction often takes place. Thus, for
instance, water on which sodium amalgam is acting contains hydrogen
in a nascent state. The hydrogen is here evolved from a liquid, and at
the first moment of its formation it must be in a condensed form.44
Sulphuric acid is poured into the space between the bell and the sides of the outer glass
cylinder, and will thus compress the gas in the bell. If the cock of the cylinder be
opened the gas will escape by it, and will be replaced by the acid, which, coining into
contact with the zinc, evolves hydrogen, and it will escape through the cock. If the
cock be closed, then the hydrogen evolved will increase the pressure of the gas in the
bell, and thus again force the acid into the space between the bell and the walls of the
outer cylinder. Thus the action of the acid on the zinc may be stopped or started at
will by opening or shutting the cock, and consequently a stream of hydrogen may be
always turned on. Now, if a piece of spongy platinum be placed in this stream, the
hydrogen will take light, because the spongy platinum becomes hot in condensing the
hydrogen and inflames it. The considerable rise in temperature of the platinum depends,
among other things, on the fact that the hydrogen condensed in its pores comes into
contact with previously absorbed and condensed atmospheric oxygen, with which hydrogen
combines with great facility in this form. In this manner the hydrogen cigar-light gives
a stream of burning hydrogen when the cock is open. In order that it should work
regularly it is necessary that the spongy platinum should be quite clean, and it is best
enveloped in a thin sheet of platinum foil, which protects it from dust. In any case,
after some time it will be necessary to clean the platinum, which may be easily done by
boiling it in nitric acid, which does not dissolve the platinum, but clears it of all
dirt. This imperfection has given rise to several other forms, in which an electric
spark is -made to pass before the orifice from which the hydrogen escapes. This is
arranged in such a manner that the zinc of a galvanic element is immersed when
the cock is turned, or a small coil giving a spark is put into circuit on turning the
hydrogen on.
45 Under conditions the same as those in which hydrogen combines with oxygen it is
also capable of combining with chlorine. A mixture of hydrogen and chlorine explodes
on the passage of an electric spark through it, or on contact with an incandescent sub
stance, and also in the presence of spongy platinum ; but, besides this, the action of light
alone is enough to bring about the combination of hydrogen and chlorine. If a mixture
of equal volumes of hydrogen and chlorine be exposed to the action of sunlight, com-
plete combination rapidly ensues, accompanied by a report. Hydrogen does not combine
directly with carbon, neither at the ordinary temperature nor by the action of heat and
pressure. But if an electric current be passed through carbon electrodes at a short
distance from each other (as in the elecric light or voltaic arc), so as to form an electric
arc in which the particles of carbon are carried from one pole to the other, then, in the
intense heat to which the carbon is subjected in this case, it is capable of combining
with hydrogen. A peculiar-smelling gas, called acetylene, C.,H..>, is thus formed from
carbon and hydrogen.
44 There is another explanation for the facility of the reactions which proceed at the
moment of separation. We shall afterwards learn that the molecule of hydrogen contains
two atoms, H2, but there are elements the molecules of which only contain one atom —
for instance, mercury. Therefore, every reaction of gaseous hydrogen must be accom-
panied by the dissolution of that bond which exists between the atoms forming a mole-
cule. At the moment of evolution, however, it is supposed that free atoms exist, and
for this reason, according to the hypothesis, act energetically. This hypothesis is not
borne out by facts, and the conception of hydrogen being condensed at the moment of
its evolution is more natural, and is in accordance with the fact (Note 12) that com-
pressed hydrogen displaces palladium and silver (Brunner, Beketoff) — that IP, acts as at
the moment of its evolution.
THE COMPOSITION OF WATER, HYDROGEN 145
In this condensed form it is capable of reacting on substances on which
it does not act in a gaseous state. There is a very intimate and evident
relation between the phenomena which take place in the action of
spongy platinum and the phenomena of the action in a nascent state.
The combination of hydrogen with aldehyde may be taken as an ex-
ample. Aldehyde is a volatile liquid with an aromatic smell, boiling at
21°, soluble in water, and absorbing oxygen from the atmosphere, and
in this absorption forming acetic acid — the substance which is found in
ordinary vinegar. If sodium amalgam be thrown into an aqueous
solution of aldehyde, the greater part of the hydrogen evolved combines
with the aldehyde, forming alcohol — a substance which is also soluble
in water, which forms the principle of all spirituous liquors, boils at 78°,
and which contains the same amount of oxygen and carbon as aldehyde,
but more hydrogen. The composition of aldehyde is C2H,0, and of
alcohol C2H6O. Reactions of substitution or displacement of metals
by hydrogen at the moment of its evolution are particularly nume-
rous.4"'
Metals, as we shall afterwards see, are in many cases able to replace
each other ; they also, and in some cases still more easily, replace and
are replaced by hydrogen. We have already seen examples of this in
the formation of hydrogen from water, sulphuric acid, ttc. In all these
cases the metals sodium, iron, or zinc displace the hydrogen which occurs
in these compounds. Hydrogen may be displaced from many of its
compounds by metals by exactly the same method as it is displaced
45 When, for instance, an acid and zinc are added to a salt of silver, the silver is
reduced ; but this may be explained as a reaction of the zinc, and not of the hydrogen at
the moment of its evolution. There are, however, examples to which this explanation
is entirely inapplicable ; thus, for instance, hydrogen, at the moment of its evolution,
easily takes up oxygen from its compounds with nitrogen if they be in solution, and
converts the nitrogen into its combination with hydrogen. Here the nitrogen and hydrogen,
so to speak, meet at the moment of their evolution, and in this state combine together.
It is evident from this that the elastic gaseous state of hydrogen fixes the limit of its
energy : hinders it from entering into those combinations of which it is capable. In the
nascent state we have hydrogen which is not in a gaseous state, and its action is then
much more energetic. This is rendered very clear from the conception of chemical
energy, because the process of passing into a gas requires a certain amount of heat, and
consequently absorbs a certain amount of work. If gaseous hydrogen is produced, it
shows that there are already conditions sufficient for the transmission of heat to the
hydrogen evolved in order to convert it into a gas. It is evident at the moment of evo-
lution that heat, which would be latent in the gaseous hydrogen, is transmitted to its
molecules, and consequently they are in a state of potential, and can hence act on many
substances.
Let us here remark the circumstance, which will be clearly understood from what has
been said above, that hydrogen condensed in the pores of certain metals, like palladium
and platinum, acts as a reducing agent on many substances. It will afterwards be
understood that substances containing much hydrogen, and easily parting with it, can
also act vigorously in effecting a reduction.
VOL. I. L
146 PRINCIPLES OF CHEMISTRY
from water ; so, for example, hydrochloric acid, which is formed
directly by the combination of hydrogen with chlorine, gives hydrogen
by the action of a great many metals, just as sulphuric acid does.
Potassium and sodium also displace hydrogen from its compounds with
nitrogen ; it is only from its compounds with carbon that hydrogen is
not displaced by metals. Hydrogen, in its turn, is able to replace
metals ; this is accomplished most easily on heating, and with those
metals which do not themselves displace hydrogen. If hydrogen be
passed over the compounds of many metals with oxygen at a red heat,
it takes up the oxygen from the metals and displaces them just
as it is itself displaced by metals. If hydrogen be passed over the
compound of oxygen with copper at a red heat, then metallic copper
and water are obtained — CuO-fH2=H2O + Cu. This kind of double
decomposition is called reduction with respect to the metal, which is
thus reduced to a metallic state from its combination with oxygen.
But it must be recollected that all metals do not displace hydrogen
from its compound with oxygen, and, conversely, hydrogen is not able
to displace all metals from their compounds with oxygen ; thus it does
not displace potassium, calcium, or aluminium from their compounds
with oxygen. If the metals be arranged in the following series :
K, Na, Ca, Al . . . . Fe, Zn, Hg . . . . Cu, Pb, Ag, Au, then
the first are able to take up oxygen from water — that is, displace
hydrogen — whilst the last do not act thus, but are, on the contrary,
reduced by hydrogen — that is, have, as is said, a less affinity for
oxygen than hydrogen, whilst potassium, sodium, calcium have more.
This is also expressed by the amount of heat evqlved in the act of
combination with oxygen, and is shown by the fact that potassium and
sodium and other similar metals evolve heat in decomposing water : but
copper, silver, and the like do not do this, because in combining with
oxygen they evolve less heat than hydrogen does, and therefore it hap-
pens that when hydrogen reduces these metals heat is evolved. Thus,
for example, if 16 grams of oxygen combine with copper, 38000 units of
heat are evolved ; and when 16 grams of oxygen combine with hydrogen,
forming water, 69000 units of heat are evolved ; whilst 23 grams of
sodium, in combining with 16 grams of oxygen, evolve 100000 units of
heat. This example clearly shows that chemical reactions which pro-
ceed directly and unaided evolve heat. Sodium decomposes water and
hydrogen reduces copper, because they are exothermal reactions, or
those which evolve heat ; copper does not decompose water, because
such a reaction would be accompanied by an absorption (or secretion)
of heat, or belongs to the class. of endothermal reactions, in which heat
is absorbed ; and such reactions do not generally proceed directly,
Till: COMPOSITION OF WATER, HYDROGEN 147
although they may take place with the aid of energy (electrical, ther-
mal, &c.) borrowed from some foreign source."1
The reduction of metals by hydrogen is taken advantage of for
determining the exact composition of water by weight. Copper oxide is
usually chosen for this purpose. It is heated to redness in hydrogen,
and the quantity of water thus formed is determined, then the quantity
of oxygen which occurs in it is found from the loss in weight of the
copper oxide. This loss will depend on the fact that the oxygen has
entered into the water. The copper oxide must be weighed immediately
before and after the experiment. The difference shows the weight of
the oxygen which entered into the composition of the water formed.
In this manner only solids have to be weighed, which is a very great
gain in the accuracy of the results obtained.47 Dulong and Berzelius
(1819) were the first to determine the composition of water by this
method, and they found that water contains 88'91 of oxygen and 11*09
of hydrogen in 100 parts, or 8-008 parts of oxygen per one part of
hydrogen. Dumas (1842) improved on this method,48 and found that
46 Several numerical data and reflections bearing on this matter are enumerated in
Notes 7, 9, and 11. It must be observed that the action of iron or zinc on water, or, con-
versely, of hydrogen on the oxides of iron or zinc, forms a reversible reaction, which
proceeds in one or the other direction, according to which is removed from the sphere of
action ; the hydrogen or the water act according to which is present in a predominating
mass. The influence of mass is clearly evinced in this case. . But the reaction
CuO + H.2 = Cu + HoO is not reversible ; the difference between the degrees of affinity is
very great in this case, and, therefore, as far as is at present known, no hydrogen is
evolved even in the presence of a large excess of water. It is to be further remarked,
that under the conditions of the dissociation of water, copper is not oxidised by water, most
probably because the oxide of copper itself is decomposable by heat.
47 This determination may be carried on in an apparatus like that mentioned in Note
13 of Chapter I.
48 We will proceed to describe Dumas' method and results. For this determination
pure and dry copper oxide is necessary. Dumas took a sufficient quantity of copper
oxide for the formation of 50 grams of water in each determination. As the oxide of
copper was weighed before and after the experiment, and as the amount of oxygen con-
tained in water was determined by the difference between these weights, it was essential
that no other substance besides the oxygen forming the water should be evolved from
the oxide of copper during its ignition in hydrogen. It was necessary, also, that the
hydrogen should be perfectly pure, and free not only from traces of moisture, but from
any other impurities which might dissolve in the water or combine with the copper and
form some other compound with it. The bulb containing the oxide of copper (fig. 26),
and which was heated to redness, should be quite free from air, as otherwise the oxygen
in the air might, in combining with the hydrogen passing through the vessel, form water
in addition to the oxygen of the oxide of copper. The water formed should be entirely
absorbed in order to accurately determine the quantity of the resultant water. The
hydrogen was evolved in the three-necked bottle. The sulphuric acid, for acting on the zinc,
is poured through funnels into the middle neck. The hydrogen evolved in the Woulfe's
bottle passes through U tubes, in which it is purified, to the bulb, where it comes into
contact with the copper oxide, forms water, and reduces the oxide to metallic copper;
the water formed is condensed in the second bulb, and any passing off is absorbed in the
second set of U tubes. This is the general arrangement of the apparatus. The bulb
L 2
148
PRINCIPLES OF CHEMISTRY
water contains 12 '575 parts of hydrogen per 100 parts oxygen, that is —
7-990 parts of oxygen per 1 part of hydrogen, and therefore it is usually
with the copper oxide is weighed before and after the experiment. The loss in weight,
shows the quantity of oxygen which went into the composition of the water formed,
the weight of the latter being
shown by the gain in weight of
the absorbing apparatus. Know-
ing the amount of oxygen in the
water formed, we also know the
quantity of hydrogen contained
in it, and consequently we deter-
mine the composition of water by
weight. This is the essence of the
determination. We will now turn
to particulars. In one neck of the
three-necked bottle there is placed
a tube immersed in mercury. This
serves as a safety-valve to pre-
vent the pressure inside the ap-
paratus becoming too great from
the rapid evolution of hydrogen.
Did the pressure rise to any con-
siderable extent, the current of
gases and vapours would be very
rapid, and, as a consequence, the
hydrogen would not be perfectly
purified, or the water be entirely
absorbed in the tubes placed for
this purpose. In the third neck
of the Woulfe's bottle there is a
tube leading the hydrogen to the
purifying apparatus, consisting
of eight U tubes, destined for the
purification and testing of the hy-
drogen. The hydrogen, evolved
by zinc and sulphuric acid, is
purified by passing it first through
a tube full of pieces of glass moist-
ened with a solution of lead ni-
trate, next through silver sul-
phate; the lead nitrate retains
sulphuretted hydrogen, and ar-
seniuretted hydrogen is retained
by the tube with silver sulphate.
Caustic potash in the next U tube
retains any acid which might
come over. The two follow-
ing tubes are filled with lumps of
dry caustic potash in order to ab-
sorb any carbonic anhydride and
moisture which the hydrogen
might contain. The next two tubes
are, to completely dry the gas,
filled with a powder of phosphoric
THE COMPOSITION OF WATER, HYDROGEN 149
received that -i rater contains eight parts by weight of oxygen per one part
/ii/ /n •!(//! f. of hydrogen. By whatever method water be obtained, it will
anhydride, intermingled with lumps of pumice-stone. They are immersed in a freezing
mixture. The small U tube contains hygroscopic substances, and is weighed before the
experiment : this is in order to know whether the hydrogen passing through still retains
any moisture. If it does not, then the weight of this tube will not vary during the
whole experiment, but if the hydrogen evolved still retains moisture, the tube will in-
crease in weight. The copper oxide is dropped into the bulb, which is, previous to the
experiment, dried with the copper oxide during a long period of time. The air is
then exhausted from it, in order to weigh the oxide of copper in a vacuum and to
avoid making any correction for weighing in air. The bulb is made of infusible glass,
that it may be able to withstand a lengthy (20 hours) exposure to a red heat without
changing in form. The weighed bulb is only connected with the purifying apparatus after
the hydrogen has already passed through for a long time, and after experiment has shown
that the hydrogen passing from the purifying apparatus is pure and does not contain
any air. When the bulb is connected with the purifying apparatus, its cock is opened
and the hydrogen fills the bulb. The drawn-out end of the bulb is joined by an india-
rubber tube with the second bulb, in which the water formed is condensed. When this
connection is made, the thread binding up the india-rubber tube is untied, and then the
hydrogen can pass freely through the apparatus. On passing from the condensing bulb
the gas and vapour enter into an apparatus for absorbing the last traces of moisture.
The first U tube contains pieces of ignited potash, the second and third tubes phosphoric
anhydride or pumice-stone moistened with sulphuric acid. The last of the two is
employed for determining whether all the moisture is absorbed, and is therefore weighed
separately. The final tube only serves as a safety-tube for the whole apparatus, in order
that the external moisture should not penetrate into it. The glass cylinder contains
sulphuric acid, through which the excess of hydrogen passes; it enables the rate at
which the hydrogen is evolved to be judged, and whether its amount should be decreased
or increased.
When the apparatus is set up it must be seen that all its parts are hermetically tight
before commencing the experiment. When the previously weighed parts are joined up
together and the whole apparatus put into communication, then the bulb containing the
copper oxide is heated with a spirit lamp (reduction does not take place without the aid
of heat), and the reduction of the copper oxide then takes place, and water is formed,
which condenses in the absorbing apparatus. When nearly all the copper oxide is re-
duced the lamp is removed and the apparatus allowed to cool, the current of hydrogen
being kept up all the time. When cool, the drawn-out end of the bulb is fused up, and
the hydrogen remaining in it is exhausted, in order that the copper may be again weighed in
a vacuum. The absorbing apparatus remains full of hydrogen, and would therefore present
a less weight than if it were full of air, as it was before the experiment, and, therefore,
having disconnected the copper oxide bulb, a current of dry air is passed through it until
the gas passing from the glass cylinder is quite free from hydrogen. The condensing
bulb and the two tubes next to it are then weighed, in order to determine the quantity of
water formed. Dumas repeated this experiment many times. The average result was
that water contains 1253'3 parts of hydrogen per 10000 parts of oxygen. Making a
correction for the amount of air contained in the sulphuric acid employed for producing
the hydrogen, Dumas obtained the average figure 1251'5, between the extremes 1247 -2
a n< I 1256-2. This proves that per 1 part of hydrogen water contains 7'9904 parts of
oxygen, with a possible error of not more than 7^, or 0'08, in the amount of oxygen per
1 part of hydrogen.
Erdmann and Marchand, in eight determinations, found that per 10000 parts of
oxygen water contains an average of 1252 parts of hydrogen, with a difference of from
1258-5 to 1248-7 ; hence per 1 part of hydrogen there would be 7'9952 of oxygen, with an
error of at least 0'05, because, taking the figure 1258'5, the amount of oxygen per 1
part of hydrogen would be 7'944.
150 PKIXCIPLES OF CHEMISTRY
always present the same composition. Whether it be taken from nature
and purified, or whether it be obtained from hydrogen by oxidation, or
whether it be separated from any of its compounds, or obtained by some
double decomposition — it will in every case contain one part of hydrogen
and eight parts of oxygen. This is because water is a definite chemical
compound. Detonating-gas, from which it may be formed, is a simple
mixture of oxygen and hydrogen, although a mixture of the same
composition as water. All the properties of both constituent gases are
preserved in detonating-gas. Either one or the other gas may be
added to it without destroying its homogeneity. The fundamental
properties of oxygen and hydrogen are not found in water, and neither
of the gases can be added to it. But they may be evolved from it. In
the formation of water there is an evolution of heat ; for the decom-
position of water heat is required. All this is expressed by the words,
Water is a definite chemical compound of hydrogen with oxygen. Tak-
ing the symbol of hydrogen, H, as expressing a unit quantity by weight
of this substance, and by expressing 16 parts by weight of oxygen by O,
we can express all the above statements by the chemical symbol of
water, H0O. As only definite chemical compounds are denoted by
formulae, having denoted the formula of a compound substance, we
express by it the entire series of conceptions which are connected with the
representation of a definite compound, and, at the same time, the quan-
titative composition of the substance by weight. Further, as we shall
afterwards see, formulae express the volume of the gases contained in a
substance. Thus the formula of water shows that it contains two volumes
of hydrogen and one volume of oxygen. Besides which, we shall learn
that the formula expresses the density of the vapour of a compound,
and on this, as we have seen, many properties of substances depend.
This vapour density, as we shall learn, also determines the quantity of
a substance entering into a reaction. Thus the letters H2O tell
the chemist the entire history .of the substance. This is an inter-
national language, which endows chemistry with a simplicity, clear-
ness, stability, and trustworthiness founded on the investigation of the
laws of nature.
Reiser (1888), in America, by employing palladium hydride, and by introducing
various new precautions for obtaining accurate results, found the composition of water
to be 15'95 parts of oxygen per 2 of hydrogen.
Certain of the latest determinations of the composition of water are hardly less exact
than the analysis made by Dumas, and always give less than 8, and on the average
7'98, of oxygen per 1 part of hydrogen. At present, therefore, the atomic weight of
oxygen is taken as 15'96. However, this figure is not to be entirely depended on, and
for ordinary accuracy it may be considered that O = 16.
151
CHAPTER III
OXYGEN AXD THE CHIEF ASPECTS OF ITS SALINE COMBINATIONS.
ON the earth's surface there is no other element which is so widely dis-
tributed as oxygen in its various compounds.1 It makes up eight-ninths
of the weight of water, which occupies the greater part of the earth's
surface. Nearly all earthy substances and rocks consist of compounds
of oxygen with metals and other elements. Thus, the greater part of
sand is formed of silica, SiO2, which is a compound of oxygen with silicon,
and contains 53 p.c of oxygen ; clay contains water, alumina (formed of
aluminium and oxygen), and silica. It may be considered that earthy
substances and rocks contain up to one-third of their weight of oxygen ;
animal and vegetable substances are also very rich in oxygen. With-
out counting the water present in them, plants contain up to 40, and
animals up to 20 p.c. by weight of oxygen. Thus, oxygen compounds
predominate on the earth's surface, and form about one-half of the
whole of the solid and liquid matters of the earth's crust. Besides
this, a portion yet remains free, and is contained in admixture with
nitrogen in the atmosphere, forming about one-fourth of its mass, or
one-fifth of its volume.
Being so widely distributed in nature, oxygen plays a very im-
portant part in it, for a number of the phenomena which take place
before us are mainly dependent on it. Animals breathe air in order
to obtain only oxygen from it, the oxygen entering into their
respiratory organs (the lungs of human beings and animals, the gills of
fishes, and the trochae of insects) ; they, so to say, drink in air in order
to absorb the oxygen. The oxygen of the air (or dissolved in water)
passes through the membranes of the respiratory organs into the blood,
is retained in it by the blood corpuscles, is transmitted by their
means to all parts of the body, aids their transformations, bringing
1 As regards the interior of the earth, it probably contains far less oxygen compounds
than the surface, judging by the accumulated evidences of the earth's origin, of mete-
orites, of the earth's density, &c., as set forth in the fourth chapter of my work on the
' Naphtha Industry,' 1877, in speaking of the origin of naphtha.
152 PEINCIPLES OF CHEMISTRY
about chemical processes in them, and chiefly extracting carbon from
them in the form of carbonic anhydride, the greater part of which
passes into the blood, is dissolved by it, and is thrown off by the lungs
during the absorption of the oxygen. Thus, in the process of respiration
carbonic anhydride (and water) is given off, and the oxygen of the air
absorbed, by which means the blood is changed from a dark-red
venous to a bright-red arterial blood. The cessation of this process causes
death, because then all those chemical processes, and the consequent
heat and work which the oxygen introduced into the system brought
about, ceases. For this reason suffocation and death ensue in a vacuum,
or in a gas which does not contain free oxygen (which does not support
combustion). If an animal be placed in an atmosphere of free oxygen,
then at first its movements are very active and a general invigoration is
remarked, but a reaction soon sets in, and perhaps death may ensue.
The oxygen of the air, when it enters the lungs, is diluted with four
volumes of nitrogen, which is not absorbed into the system, and there-
fore the blood absorbs but a small quantity of oxygen from the air,
whilst in an atmosphere of pure oxygen a large quantity of oxygen
would be absorbed, which would produce a very rapid change of all parts
of the organism, and destroy it. From what has been said, it will be
understood that oxygen may be employed in respiration, at least for a
limited time, when the respiratory organs suffer under certain forms of
suffocation and impediment to breathing.2
The combustion of organic substances — that is, substances which
make up the composition of plants and animals- — proceeds in the
same manner as the combustion of many inorganic substances, such as
sulphur, phosphorus, iron, &c., from the combination of these sub-
stances with oxygen, as was described in the Introduction. The de-
composition, rotting, and similar transformations of substances, which
2 It is evident that the partial pressure (see Chap. II.) acts in respiration. The researches
of Paul Bert showed this with particular clearness. Under a pressure of one-fifth of an at-
mosphere consisting of oxygen only, animals and human beings remain under the ordinary
conditions of the partial pressure of oxygen, but organisms cannot support air rarefied to one-
fifth, for then the partial pressure of the oxygen falls to one-twenty-fifth of an atmosphere.
Even under a pressure of one-third of an atmosphere the regular life of human beings is im-
possible, by reason of the impossibility of respiration (of the decrease of solubility of oxygen
in the blood), owing to the small partial pressure of the oxygen, and not from the mechani-
cal effect of the decrease of pressure. Paul Bert illustrated all this by many experiments,
some of which he conducted on himself. This explains, among other things, the discom-
fort felt in the ascent of high mountains or in balloons when the height reached exceeds
eight kilometres, and at pressures below 250 mm. (Chap, II. note 23). It is evident that
an artificial atmosphere has to be employed in the ascent to great heights, just as in sub-
marine work. The cure by compressed and rarefied air which is practised in certain ill-
nesses is based partly on the mechanical action of the change of pressure, and partly on
the alteration in the partial pressure of the respired oxygen.
OXVHKN AND JTS SALINE COMBINATIONS 158
proceed around us, are also very often dependent on the action of the
oxygen of the air, and also reduce it from a free to a combined state.
The majority of the compounds of oxygen are, like water, very stable,
and do not give up their oxygen under the ordinary conditions of nature.
As these processes are taking place everywhere, therefore the amount
of free oxygen in the atmosphere should decrease, and this decrease
should proceed somewhat rapidly. This is, in fact, observed where
combustion or respiration proceeds in a closed space. Animals suffocate in
a closed space because in consuming the oxygen the air remains unfit for
respiration. In. the same manner combustion, in time, ceases in a closed
space, which may be proved by a very simple experiment. An ignited
.substance — for instance a piece of burning sulphur — has only to be placed
in a glass flask, which is then closed with a stout cork to prevent the
access of the external air ; combustion will proceed for a certain time,
so long as the flask contains any free oxygen, but it will cease, although
there still remain unburnt sulphur, when all the oxygen of the enclosed
air has combined with the sulphur. From what has been said, it is
evident that regularity of combustion or respiration requires a con-
stant renewal of air — that is, that the burning substance or respiring
animal should have access to a fresh supply of oxygen. This is attained
in human habitations by having many windows, outlets, and ventilators,
and by the current of air produced by tires and stoves. As regards the
air over the entire earth's surface, its amount of oxygen hardly decreases,
because in nature there is a process going on which renews the supply
of free oxygen. Plants, or rather their leaves, during daytime 3 — that is,
under the influence of light — evolve free oxygen. Thus the loss of
oxygen which occurs in consequence of the respiration of animals and of
combustion is made good by plants. If a leaf be placed in a bell jar con-
taining water, and carbonic anhydride (because this gas is absorbed and
oxygen evolved from it by plants) be passed into the bell, and the whole
-apparatus be placed in sunlight, then oxygen will accumulate in the
bell jar. This experiment was first made by Priestley at the end of the
last century. Thus the life of plants on the earth not only serves for
the formation of food for animals, but also for keeping up a constant
percentage of oxygen in the atmosphere. In the long period of the life of
the earth that equilibrium has been attained between the processes ab-
•" A t night, without the action of light, without the absorption of that energy which
is required for the decomposition of carbonic anhydride into free oxygen and carbon,
which is retained by the plants, they breathe like animals, absorbing oxygen and evolving
carbonic anhydride. This process also goes on side by side with the reverse process in
daytime, but then it is far feebler than that which gives oxygen. This observation is a
necessary consequence of an aggregate of data referring to the physiological processes of
plants.
154 PRINCIPLES OF CHEMISTRY
sorbing and envolving oxygen, by which a definite quantity of free
oxygen is preserved in the entire mass of the atmosphere.4
Free oxygen may be obtained by one or another method from all
the substances in which it occurs. Thus, for instance, the oxygen of
many substances may be transferred into water, from which, as we
have already seen, oxygen may be obtained.5 We will first consider
the methods of extracting oxygen from air as being a substance every-
where distributed. The separation of oxygen from it is, however,
hampered by many difficulties.
From air, which contains a mixture of oxygen and nitrogen, the
nitrogen alone cannot be removed, because it has 110 inclination to
combine directly or readily with any substance ; and although it does
combine with certain substances (boron, titanium), these substances com-
bine simultaneously with the oxygen of the atmosphere.6 However,
4 The earth's surface is equal to about 510 million square kilometres, and the mass of
the air (at a pressure of 760 mm.) on each kilometre of surface is about 10 J thousand millions
of kilograms, or about 10^ million tons ; therefore the whole weight of the atmosphere
is about 5100 million million ( = 51xl014) tons. Consequently there are about 2 x 1015
tons of free oxygen in the earth's atmosphere. The innumerable series of processes
which absorb a portion of this oxygen are compensated for by the plant processes. Count-
ing that 100 million tons of vegetable matter, containing 40 p.c. of carbon, formed from
carbonic acid, are produced (and the same process proceeds in water) per year on the 100
million square kilometres of dry land (ten tons of roots, leaves, stems, &c. per hectare, or
Y£O of a square kilometre), we find that the plant life of the dry land gives about 100,000
tons of oxygen, which is an insignificant fraction of the entire mass of the oxygen of
the air.
5 The extraction of oxygen from water may evidently be accomplished by two pro-
cesses : either by the decomposition of water into its constituent parts by the action of a
galvanic current (Chap. II.), or by means of the removal of the hydrogen from water.
But, as we have seen and already know, hydrogen enters into direct combination with very
few substances, and then only under special circumstances ; whilst oxygen, as we
shall soon learn, combines with nearly all substances. Only gaseous chlorine (and
especially, fluorine) is capable of decomposing water, taking up the hydrogen from it,
without combining with the oxygen. Chlorine is soluble in water, and if an aqueous
solution of chlorine, so-called chlorine water, be poured into a flask, and this flask be
inverted in a basin containing the same chlorine water, then we shall have an apparatus
by means of which oxygen may be extracted from water. At the ordinary temperature,
and in the dark, chlorine does not act on water, or only acts very feebly ; but under
the action of direct sunlight chlorine decomposes water, with the evolution of oxygen.
The chlorine then combines with the hydrogen, and gives hydrochloric acid, which dis-
solves in the water, and therefore free oxygen only will be separated from the liquid:
and it will only contain a small quantity of chlorine in admixture, which can be easily
removed by passing the gas through a solution of caustic potash, which retains the
chlorine.
6 A difference in the physical properties of both gases cannot be here taken advantage
of, because they are very similar in this respect. Thus the density of oxygen is 1(5, and
of nitrogen 14 times greater than the density of hydrogen, and therefore porous vessels
cannot be here employed — the difference between the times of their passage through a
porous surface would be too insignificant.
Graham, however, succeeded in enriching air in oxygen by passing it through india-
OXYGEN AND ITS SALINE COMBINATIONS
155
oxygen may be separated from air by causing it to combine with sub-
stances which may be easily decomposed by the action of heat, and, in
rubber. This may be done in the following way : — A common india-rubber cushion, E
(Fig. 27), is taken, and its orifice hermetically connected with an air-pump, or, better
still, a mercury aspirator (the Sprengel pump is designated by the letters A, c, B). "When
the aspirator (Chap. II. note 16)
pumps out the air, which will be
seen by the mercury running
out in an almost uninterrupted
stream, and from its stand-
ing at near the barometric
height, then it may be clearly re-
marked that gas passes through
the india-rubber. This is also
seen from the fact that bubbles
of gas continually pass along with
the mercury. A small pressure
of air may be constantly kept
up in the cushion by pouring
mercury into the funnel A, and
screwing up the cock c, so that
the stream flowing from it be
small, and then a portion of the
air passing through the india-
rubber will be carried along
with the mercury. This air may
be collected in the cylinder B.
Its composition proves to be
about 42 volumes of oxygen with
57 volumes of nitrogen, and one
volume of carbonic anhydride,
whilst ordinary air contains
only 21 volumes of oxygen in
100 volumes. A square metre of
india-rubber surface (of the usual
thickness) passes about 45 c.c. of
such air per hour. This experi-
ment clearly shows that india-
rubber is permeable to gases.
This may, by the way, be ob-
served in common toy balloons
filled with coal-gas. They fall
after a day or two, not be-
cause there are holes in them,
but because air penetrates into,
and the gas from, their interior,
through the surface of the india-
rubber of which they are made. The rate of the passage of gases through india-
rubber does not, as Mitchell and Graham showed, depend on their densities, and con-
sequently its permeability is not determined by orifices. It more resembles dialysis
— that is, the penetration of liquids through colloid surfaces. Equal volumes of gases
penetrate through india-rubber in periods of time which are related to each other as
follows : — carbonic anhydride, 100 ; hydrogen, 247 ; oxygen, 582 ; marsh gas, 688 ; carbonic
oxide, 1220 ; nitrogen, 1858. Hence nitrogen penetrates more slowly than oxygen, and
carbonic anhydride more quickly than other gases. 2' 556 volumes of oxygen and
Fra> 27.-Graham's apparatus for the decomposition of air
by pumping it through india-rubber.
-o lining. u'ive up the oxygen absorbed that is, l»v making use of re-
versible react io] i v. 'llms. ft>i' instance, the oxv^'en < »f t he at niosphere
may In- made to oxidise sulphurous anhydride, S( )., (bypassing directly
over ignited spongy platinum), and to form sulphuric1 anhydride, or
sulphur trioxide. S( );j : and this su list a net1 (which is a solid and volatile,
and therefore, easily separated from the nitrogen and sulphurous
anhydride), l»y heating again, gives oxvgen and sulphurous anhydride.
Caustic >oda or lime extracts (absorbs) the sulphurous anhydride from
this mixture, whil>t the oxygen is not absorbed, and thu> it is isolated
from the air. < hi a lar^e scale in works, as we >hall afterwards see,
sulphurous anhydride is transformed into hydrate of .-ulphuric tri oxide,
or sulphuric acid. H._,S(),: if this is made to fall in drops on reddiot
flagstones, water, sulphurous anhydride, and oxygen are obtained.
The oxygen i^ ra-ilv isolated from this mixture bv parsing the gases
over lime. The extraction of oxvgen from oxide of mercury
(Priestley, Lavoisier;, which is obtained from meivurv and the oxvgen
ot the atmosphere, is also a reversible reaction bv which oxygen mav be
obtained from the atmosphere. So also, bv passing diy air through a
red-hot tube containing barium oxide, it is made- to combine with tin
oxygen of the air. \}y this reaction the so-called barium peroxide.
I»a< ) „ is formed from the barium oxide I>a() and -at a higher tempe-
rature the former evolves the absorbed oxygen, and leaves the bari
oxide originally taken.7
[f the process of dialv-i- 1"' repealed on the ;iir \vlucl
i i ndia- ruhl icr. then a mixture coiitainin.;' i'.,"> p.c. l>y volnnn
uiy lie thouudit that the cause ol this phen. nneiimi i- the ah
.11 -,, Chap. 1 1 of :_:ases l,y india-ruhher and the ,.\ ,,liH imi of the -a-
iim : and. indeed, india rnld'cr does ali-orli ptsex, especially carlimiii
metal-, especially mi an increase of temperature. al>-<>H> ^ases, as \va-
ipter. (iraham called the ahove method of the decompositi. i
; Ti e preparation ot oxygen li\ tin- method, uhidi is due to !'.nn-.en. i- conducted ii
.1 porcelain tnl.e. whicli i- placed in a stove heated liv charcoal, -o that it- emU project
pn dried ; ed in the tnl.e. one end of \\ Inch i> cmuiecteil \\ ith a pair ci
:. d< . Mr keeping up a current of air tliroii-h it. The air is previmish
pa ed t h)-oii. _d i .1 -olnt ion i ,f can-.! ic pota-h. to renio\-e all t race- it carl ionic anhydride
a .1 ' er> caret, ill;, rii-ie,] t . H' 1 1 1C hy d 1M t e 1 5a I I ,< ) , doe> not - i S e t he p, ToX ide I. At;
• • :, -on tin , de ot liai-iuni ah-orlis o.xy_ren from the air. so that the j.ra!
• • • im-t i-nl i'el\ of nil i-o-en. Wln-n I he a li~orpt ion cea--es, t he ail
i • - • Thekn inn i,\i.|e i-, ciiiix erled into pei'ovide under tlie~e circumstances
' i- all o I'll all. .lit one part of o\\ -en l.\ Wei-lit. When tin
,1, ..,,,•,,: '. ., . , ., , .... elided, a em-k with a •ja-cmidiictiii'_' tiihe istixiM
<>XY<;KN AND ITS SALINE COMBINATIONS 157
( >.\ygen is evolved with particular ease by a whole series of un-
stable oxygen compounds, of which we will proceed to take a general
survey, remarking that many of these reactions, although not all, belong
to the number of reversible reactions ; 8 so that in order to ob-
tain many of these substances (for instance, potassium chlorate) rich
in oxygen, recourse must be had to indirect methods (see Intro-
duction), with which we shall become acquainted in the course of this
book.
1. The- compounds of oxygen with certain metals, and especially
with the so-called noble metals — that is, mercury, silver, gold, and
platinum — having been once obtained, retain their oxygen at the ordi-
nary temperature, but part with it at a red heat. The compounds are
solids, generally amorphous and infusible, and are easily decomposed by
heat into the metal and oxygen. We have seen an example of this in
speaking of the decomposition of mercury oxide. Priestley, in 1774,
obtained pure oxygen for the first time by heating mercury oxide by
means of a burning-glass, and clearly showed its difference from air.
He showed its characteristic property of supporting combustion ' with
remarkable vigour,' and named it dephlogisticated air.
into the other end, and the heat of the stove is increased to a bright-red heat (800°). At
this temperature the barium peroxide gives up all that oxygen which it acquired at a dark-
red heat — i.e., about one part by weight of oxygen is evolved from twelve parts of barium
peroxide. After the evolution of the oxygen there remains the barium oxide which was
originally taken, so that air may be again passed over it, and thus the preparation of oxygen
from one and the same quantity of barium oxide may be repeated many times. Oxygen
has been procured one hundred times from one mass of oxide by this method ; all the neces-
sary precautions being taken, as regards the temperature of the mass and the removal of
moisture and carbonic acid from the air. Unless these precautions be taken, the mass
of oxide soon spoils.
As oxygen may become of considerable technical use, from its capacity for giving
high temperatures and intense light in the combustion of substances, its preparation
directly from air by practical methods forms a problem whose solution many investi-
gators continue to work at up to the present day. The most practical method is that of
Tessie du Motoy. It is based on the fact that a mass of equal weights of manganese
peroxide and caustic soda at an incipient red heat (about 850°) absorbs oxygen from air,
with the separation of water, according to the equation MnO.j + 2NaHO + O = Na.»MnO4
+ H._,O. If superheated steam, at a temperature of about 450°, be then passed through
the mixture, the manganese peroxide and caustic soda orginally taken are regenerated, and
the oxygen held by them is evolved, according to the reverse equation Na.^MnO4
+ H.jO = MnOo + '2NaHO + O. This mode of preparing oxygen may be repeated for an
infinite number of times. The oxygen in combining separates out water, and steam,
acting on the resultant substance, evolves oxygen. Hence all that is required for the
preparation of oxygen by this method is fuel and the alternate cutting off the supply of
air and steam.
8 Even the decomposition of manganese peroxide is reversible, and it may be re-
ol.tained from that suboxide (or its salts), which is formed in the evolution of oxj'gen
(Chap. XI. note 6). The compounds of chromic acid containing the trioxide CrO5 in
evolving oxygen give chromium oxide, Cr.,O3, but they re-form the salt of chromic acid
when heated at a red heat in air with an alkali.
ir>8
-. Tin- SUOM nice- called !„ •!-<>. i-'nl' x'-' eyohe oxygen at a e;reater o
le>s heat (and also by the action of many acids). They usually contaii
nift;ils combined with a laruv quantity of oxygen. Peroxides arc tin
hiu'he-t oxides df certain metals ; those metals \vhidi form them irene
rally Lfive seyeral compounds with oxygen. Those of tin- lowe-t decree
of oxidation, containing the least amount of oxygen, are generally sub
stances which are capable of easily reacting on acids for instance
with sulphuric, acid. Such low oxides art4 called bases. Peroxide:
contain more oxygen than the ba-es formed by the same metals. Fo
example, lead oxide contains 7'1 parts of oxygen in 1 < >< I parts, and i:
basic, but lead peroxide contains ]'.}•'.} parts of oxygen in lou parts
^^<t ii'/<i in x>< jn'i'n.i'n.Ji' is a similar substance, which is a solid of a darl-
colour, and occurs in nature. It is employed in the manufacture!
under the name of black oxide of manganese (in (lerman. ' Braunstein,
the pyrolusite of the mineralogist). Peroxides are able to eyolyt
oxygen at a more or less elevated temperature. They do not then pan
with all their oxygen, but with only a portion of it. and are c<>nyerte<
into a lower oxide or base. Thus, for example, lead peroxide, on heat-
in U\ u'ives oxygen and lead oxide. The decomposition of this peroxidt
proceeds Somewhat easily on heating, eyen in a glass vessel, but manga-
nese peroxide <»nl\" exolyes oxygen at a strong red heat, and therefore
oxygen can only be obtained from it in iron, or other metallic, or clay
yessels. This used to be the method for obtaining oxygen. .Man^'anest!
peroxide only ]>arts with one-third of its oxygen (accordiiiiLj to the
equation .">.M n().,= M n:(( ), + ().,), whilst t wo-tliirds remain in the solid
substance which forms the residue, from the heating. Metallic peroxides
are also capable of eyolvimj; oxygen on heating with sulpliuric acid.
'J'hey then e\'ol\e so much oxygen as is in excess of that necessary tor
the formation of the base, the latter reacting on the >ulplmric acid
forming a compound (salt) with it. Thus barium peroxide, when
heated with sulphuric acid, forms ox vgen and barium oxide, which gives
a compound with sulphuric acid which is termed barium sulphate
(BaO.J+H^SOI^l>aS()1-f-H./)-r-O). This reaction usually proceeds
with irreater ease than the decomposition of peroxides by heat
a lone. Kor t lie purposes of experiment powdered man^am-M- peroxide is
usually taken and mixed with strong sulphuric acid in a lla.-d<. and the
apparatti- set up a- -ln>wn in Fig. L'S. rl'he gas \\-hicli is e\ol\ed is
OXYGEN AND ITS SALINK COMBINATIONS
159
passed through a Woulfe's bottle containing a solution of caustic potash,
to purify it from carl ionic anhydride and chlorine, which accompany the
evolution of oxygen from commercial manganese peroxide, and the ua- is
not collected until a thin smouldering taper placed in front of the escape
orifice bursts into flame, which shows that the gas coming off is oxygen.
By this method of decomposition of the manganese peroxide by sul-
FIG. 28.— Preparation of oxygen from manganese peroxide and sulphuric' acid. The gas evolved
is passed through a Woulfe's bottle containing caustic potash.
phuric acid there is evolved, not, as in heating, one-third, but one-half
of the oxygen contained in the peroxide (Mn02 + H2S04 = MnS04 -f
HoO + O) — that is, from 50 grams of peroxide about 7i grams, or
about 5^ litres, of oxygen,10 whilst by heating only about 3^ litres are
obtained. The chemists of Lavoisier's time generally obtained oxygen
by heating manganese peroxide. Now there are more convenient
methods known.
3. A third source to which recourse may be had for obtaining
oxygen is represented in acids and salts containing much oxygen, and
which are capable, by parting with a portion or all of their oxygen,
of being converted into other compounds (lower products of oxida-
tion) which are more difficultly decomposed. These acids and salts
(like peroxides) evolve oxygen either on heating alone, or when
heated with some other substance. Sulphuric acid may be taken
as an example of an acid which is decomposed by the action of heat
alone,11 for it breaks up at a red heat into water, sulphurous anhydride,
10 Scheele, in 1785, discovered the method of obtaining oxygen by treating manganese
peroxide with sulphuric acid.
11 All acids rich in oxygen, and especially those whose elements form lower oxides,
evolve oxygen either directly at the ordinary temperature (for instance, ferric acid), or on
hciiting (for instance, nitric, manganic, chromic, chloric, and others), or if basic lou.-r
oxides are formed from them, by heating with sulphuric acid. Thus the salts
160 PRINCIPLES OF CHEMISTRY
and oxygen, as was mentioned before. Priestley, in 1772, and Scheele,
somewhat later, obtained oxygen by heating nitre to a red heat. The
best examples of the formation of oxygen by the heating of salts is given
in jwtassium chlorate, or Berthollet's salt, so called after the French
chemist who discovered it. Potassium chlorate is a salt composed of
the elements potassium, chlorine, and oxygen, KC103. It occurs as-
transparent colourless plates, is soluble in water, especially in hot
water, and resembles common table salt in some of its physical properties;
it melts on heating, and in melting begins to decompose, evolving oxygen
gas. This decomposition ends in all the oxygen being evolved from
the potassium chlorate, potassium chloride being left as a residue, accord-
ing to the equation KC1O3=KC1 + O3.12 This decomposition proceeds
at a temperature which allows of its being conducted in a vessel
made of glass. However, in decomposing, the molten potassium
chlorate swells up and boils, and gradually solidifies, so the evolution of
the oxygen is not regular, and the glass vessel may crack. In order
to overcome this inconvenience, the potassium chlorate is crushed
and mixed with a powder of a substance which is incapable of com-
bining with the oxygen evolved, and which is a good conductor of heat.
Usually it is mixed with manganese peroxide.13 The decomposition of
the potassium chlorate is then considerably facilitated, and proceeds at
a lower temperature (because the entire mass is then better heated,
both externally and internally), without swelling up, and is therefore
more convenient than the decomposition of the salt alone. This
method for the preparation of oxygen is very convenient ; it is generally
employed when a small quantity of oxygen is required. Further, potas-
sium chlorate is easily obtained pure, and it evolves much oxygen. 100
grams of the salt give as much as 39 grams, or 30 litres, of oxygen.
This method is so simple and easy,14 that a course of practical chemistry
of chromic acid (for instance, potassium dichromate, K.)Cr.)O7) give oxygen with
sulphuric acid ; first potassium sulphate, K.^SO.^ is formed, and then the chromic acid set
free gives a sulphui'ic acid salt of the lower oxide, Cr.,05.
12 This reaction is not reversible, and is exothermal — that is, it does not absorb heat,
but, on the contrary, evolves 9713 calories per molecular weight KC1O5, equal to 122
parts of salt (according to the determination of Thomsen, who burnt hydrogen in a
calorimeter either alone or with a definite quantity of potassium chlorate mixed with
oxide of iron). It does not proceed at once, but first forms perchlorate, KCIO^ (see
Chlorine and Potassium). It is to be remarked that potassium chloride melts at 788°,
potassium chlorate at 372°, and potassium perchlorate at 010°.
13 The peroxide does not evolve oxygen in this case. It may be replaced by many oxides
— for instance, by oxide of iron. It is necessary to take the precaution that no combustible
substances (such as bits of paper, splinters, sulphur, &c.) fall into the mixture, as they
might cause an explosion.
14 The decomposition of a mixture of melted and well-crushed potassium chlorate
()XV(iKN AND ITS SALINE COMBINATIONS 101
is often commenced by the preparation of oxygen by this method, and
of hydrogen by the aid of zinc and sulphuric acid, all the more as
thi'M- -uses enable many interesting and striking experiments to be
made.15
A solution of bleaching powder, which contains calcium hypo-
chlorite, CaCl2O2, evolves oxygen when gently heated with the ad-
dition of a small quantity of certain oxides — for instance, cobalt
oxide, which in this case acts by contact (see Introduction). Of
itself, a solution of bleaching powder does not evolve oxygen when
heated, but it oxidises the cobalt oxide to a higher degree of oxidation ;
this higher oxide of cobalt in contact with the bleaching powder, decom-
poses into oxygen and lower oxidation products, and the resultant lower
oxide of cobalt with bleaching powder again gives the higher oxide,
which again gives up its oxygen, and so on.16 The calcium hypo-
chlorite is here decomposed according to the equation CaCl2O2 =
CaCl2 + O2. In this manner a small quantity of cobalt oxide17 is
sufficient for the decomposition of an indefinitely large quantity
of bleaching powder.
with powdered manganese peroxide proceeds at so low a temperature (the salt does not
melt) that it may be effected in an ordinary glass flask. As the reaction is exothermal, the
decomposition of potassium chlorate with the formation of oxygen may probably be
accomplished, under certain conditions (for example under contact action), at very low
temperatures. Substances mixed with the potassium chlorate probably act partially in
this manner.
15 Many other salts evolve oxygen by heat, like potassium chlorate, but they only
part with it either at a very strong heat (for instance, common nitre) or else are un-
suited for use on account of their cost (for instance, potassium manganate), or evolve
impure oxygen at a high temperature (for instance, zinc sulphate at a red heat gives
a mixture of sulphurous anhydride and oxygen), and are not therefore used in prac-
tice.
1(1 Such is, at present, the only possible method of explaining the phenomenon
of contact action. In many cases, as here, it is supported by observations based on facts.
Thus, for instance, it is known, as regards oxygen, that often two substances rich in
oxygen retain it so long as they are separate, but directly they come into contact
free oxygen is evolved from both of them. Thus, an aqueous solution of hydrogen
peroxide (containing twice as much oxygen as water) acts in this manner on silver oxide
(containing silver and oxygen). This reaction takes place at the ordinary temperature,
and the oxygen is evolved from both compounds. To this class of phenomena may be
also referred the fact that a mixture of barium peroxide and potassium manganate with
water and sulphuric acid evolves oxygen at the ordinary temperature. It would seem
that the essence of phenomena of this kind is entirely and purely a property of
contact ; the distribution of the atoms is changed by contact, and if the equilibrium be
unstable it is destroyed. This is especially clear for substances which change exother-
inally — that is, for those reactions which are accompanied by an evolution of heat. The
decomposition CaCLO.^ = CaCL> + O.2 belongs to this class (like the decomposition of
potassium chlorate).
17 Generally a solution of bleaching powder is alkaline (contains free lime), and, there-
VOL. I. M
Ki-J
'1 i'1 /'/'"/"/'//. s «/ ti.i-i/t/i //.' s — It is a ] lerma iH'iii o-a> -tliat is, it can-
not In- liquefied by pressure at the ordinary t empcrat ure, and further,
i- only li<|Uelird with difficulty (although more easily than hydrogen) at
temporal ures below — 1 '_'<)•. because this i~> its absolute boiling point.
As its critical pressure ''•' is about ')() atmospheres, it can lie easily
1 ii] netied HIM ler prosnres ureat pr than -^ ' atmospheres at temperat ures
belo\y — 1 L!U . 1 Met cT obtained liquid oxygen at — 1 I1 ' . l>v employing a
pre»ure above 100 atmospheres. According t() I*ewar, the density <>f
«»\y^cn in a critical stale is ()•().") (\\-atcr=l ), Inn it. like all (•ilici1 sub-
>taiH't'.s in tliis Mate.-'" varies considerablv in clen>itv \\'itli a clianicc <>t'
] ircs>tn-f and t cnijieraturc, and therefore inanv in \ cst i^'ators \vlio made
their observations tiudei1 hi^li ]>ressiii'c-> ^i\-e a ^i-eatei- density, as much
as I'l. ( >.\yu'e]i, like all u'ase^, is transparent, and like the majority of
u'a>e>, colourless. It has no smell or taste, which is evident from the
tact of it-- lieinu1 a component of air. The weight of one cubic centi-
metre in grains at U and 7(50 mm. pressure is (i-(K)l l^'.is Drains, and a
litre weighs Tll'liS u'rams : it is therefore ^li^htly denser than air.
Its den.Mtv in respect to air=l'10">(), and in ropect to hydrogen =1(5
(more exactly 1 .V'.ir,).-'1
'" IT musl lir rcninrl^fd that in all tin- above-cited rea
may lie prevented by the adinixtuve of substances ca]i
example, charcoal, many carbon (organic] conipoiiiiiU. ^n
lower oxi.lation product's, ^c-. These substances absorb the oxygen evolved, coinhiiu-
with it . a nd a coin] lound containin^r oxygen, but not free oxygen, is formed. Thus, if a
Lin-e of potassium chlorate and charcoal be heated, no oxyp-n is obtained, but
an explo-ion takes place from the rapid formation of ^rases rr-nlt in;j from the com-
bination of the oxygen of the pota--inm chloi-jite \\ith the charcoal.
The oxygen ol)tained by any of the abo\ c-descrilied methods is rarely -pure. It
chloride, which retain- the water. IJesides this, the oxygen nearly always contain--
-.a. f carbonic anhy<lride. and very <.f'ten small trace- of chlorine. The oxygen may
b'- treed troni the^e impni'itie-. by pas, MIL;' it tlii'oii'jh a solution of caustic potash.
Tin- i- done in \Voiflte's bottle-, a- was described in the la-t chapter. If the potassium
te be dr\ and pure, it -i\e, almosl pure oxygen. However, if the oxygen be
• •'• loi- i-e.piration in ca-es of sickness, it should be wa-hed b\ passing it thr«.n-h a
-olntion ol caustic alkali and through water. The be-t wa\ to obtain pure oxygen
d rertly. i N, take pot a- in in perch lorale i K('lO.,i. which can be well pnrilie.l and then
pun o \ \ • j en on heating.
' ' i i ice riling the absolute boil in;/ poii it. critical prr-siir*-. and on t he critical state in
i . -'I. -ee Chaji II. Note ii'.l and :', \ .
.In<l-in- IVom -ah.it ha been -,,id in Not- ::i of the ia-t clui|it«-r. and also from the
re-ult , of direct ob.ervation, if i- evidenl that all • nb-t a nee i n a crit ieal -t ale ha\ e a
• i . and I hat thev are \ er\ compn lile.
• A uatercon i-t ot 1 volume o| oxygen and -2. -, . .In me- ol hydrogen, and contain*
IT, part- b\ weight of oxy-en per '2. part-- b\ Wei'jhl ol hydrogen, it therefore alread\
. ~ from thi . that o\ \-en i Hi time- denser than h\dro'_'en. ( 'on \erselv. the com
OXYGEN AND ITS SALINE COMBINATIONS 163
In its chemical properties oxygen is remarkable from the fact that
it very easily— and, in a chemical sense, vigorously — reacts on a number
of substances, forming oxygen compounds. However, only a few
substances and mixtures of substances (for example, phosphorus, copper
with ammonia, decomposing organic matter, aldehyde, pyrogallol with
an alkali, <kc.) combine directly with oxygen at the ordinary
temperature, whilst many substances easily combine with oxygen at a
red heat, and often this combination presents a rapid chemical reaction
accompanied by the evolution of a large quantity of heat. Every
reaction which takes place rapidly, if it be accompanied by so great an
evolution of heat as to produce incandescence, is termed combustion.
Thus combustion ensues when many metals are plunged into chlorine,
or oxide of sodium or barium into carbonic anhydride, or when a spark
falls on gunpowder. A great many substances are combustible in
oxygen, and, owing to its presence, in air also. In order to start
combustion it is generally necessary22 that the combustible substance
should be brought to a state of incandescence. When once started —
i.e., when once the incandescent portion of the substance begins to
combine with oxygen — then combustion will proceed uninterruptedly
until either all the combustible substance or all the oxygen is consumed.
The continuation of the process does not require the aid of fresh
external heat, because sufficient heat23 is evolved to raise the tempeTa-
ture of the remaining parts of the combustible substance to the required
position of water by weight may be deduced from the densities of hydrogen and oxygen,
and the volumetric composition of water. This kind of mutual and opposite correction
is a method which strengthens the practical data of the exact sciences, whose
conclusions require, above all things, the greatest possible exactitude and variety of
corrections.
It must be observed that the specific heat of oxygen at constant pressure is 0'2175,
consequently it is to the specific heat of hydrogen (8'409) as 1 is to 15'6. Hence, the
specific heats are inversely proportional to the weights of equal volumes. This signifies
that equal volumes of both gases have (nearly) equal specific heats — that is, they require
an equal quantity of heat for raising their temperature by 1°. We shall afterwards con-
sider the specific heat of different substances more fully, and we will not, therefore, linger
over it at present.
Oxygen, like the majority of difficulty-liquefiable gases, is but slightly soluble
in water and other liquids. At the ordinary temperature, 100 volumes of water dissolve
about 3 volumes of oxygen, or more exactly, at 0° 4'1 vols., at 10° 8'3, and at 20° 3*0
(measuring the volumes at the same temperature as the water). From this it is evident
that water standing in air must absorb— i.e., dissolve — oxygen. This oxygen serves for
the respiration of fishes. Fishes cannot exist in boiled water, because it does not contain
the oxygen necessary for their respiration (see Chap. I.).
-- ( Vrtain substances (with which we shall afterwards become acquainted), however,
inflame of themselves in air ; for example, impure phosphuretted hydrogen, silicon,
hydride, zinc ethyl, and pyrophorus (very finely divided iron, &c.).
-•"' If so little heat is evolved that the adjacent parts are not heated to the tempera-
ture of combustion, then combustion will cease.
M 2
164
PEINCIPLES OF CHEMISTRY
degree. Examples of this are familiar to all from every-day experience.
Combustion proceeds in oxygen with greater rapidity, and is accom-
panied by a more powerful incandescence, than in ordinary air. This
may be demonstrated by a number of very convincing experiments. If
a piece of charcoal, attached to a wire and previously brought to red-
heat, be plunged into a flask full of oxygen, it rapidly burns at a white
heat — i.e., it combines with the oxygen, forming a gaseous product of
combustion called carbonic anhydride, or carbonic acid gas. This is the
same gas that is evolved in the act of respiration, for charcoal is one of
the substances which is obtained by the decomposition of all organic
substances which contain it, and in the process of respiration part of the
constituents of the body, so to speak, slowly
burn. If a piece of burning sulphur be laid on
a small cup attached to a wire and be placed
in a flask full of oxygen, then the sulphur,
which burns in air with a very feeble flame,
burns in the oxygen with a violet flame,
which, although pale, is much larger than
in air. If the sulphur be exchanged for a
piece of phosphorus,24 then, unless the phos-
phorus be heated, it combines very slowly
with the oxygen ; but, if heated, although
on only one spot, it burns with a very bril-
liant white flame, which is unbearable to
the sight. In order to heat the phosphorus
inside the flask, the most simple way is to bring a red-hot wire into con-
tact with it. Before the charcoal can burn, it must be brought to a state
of incandescence. Sulphur also will not burn under 100°, whilst phos-
phorus inflames at 40°. Phosphorus which has been already lighted in air
cannot so well be introduced into the flask, because it burns very rapidly
and with a large flame in air. If a small lump of metallic sodium be put
in a small cup made of lime,25 melted, and inflamed,26 then it burns very
feebly in air. But if burning sodium be immersed in oxygen, the
FIG. 29.— Mode of burning sul
phur, phosphorus, sodium, &c.
in oxygen
24 The phosphorus must be dry ; it is usually kept in water, as it oxidises in air. It
should be cut under water, as otherwise the freshly-cut surface oxidises. It must be dried
carefully and quickly by wrapping it in blotting-paper. If damp, it splutters in burning.
A small piece should be taken, as otherwise the iron spoon will melt. In this and the
other experiments on combustion, water should be poured over the bottom of the vessel
containing the oxygen, to prevent it from cracking. The cork closing the vessel should not
fit tightly, otherwise it may fly off with the spoon and burning substance, owing to the
expansion due to the heat of the combustion.
25 An iron cup will melt with sodium in oxygen.
26 In order to rapidly heat the lime crucible with the sodium, they are heated in the
flame of a blow-pipe described in Chap. VIII.
OXYGEN AND ITS SALINE r< >.M IM NATIONS 165
combustion is invigorated and is accompanied by a brighter yellow
flame. Metallic magnesium, which burns brightly in air, continues to
burn with still greater vigour in oxygen, forming a white powder,
which is a compound of magnesium with oxygen (magnesium oxide ;
magnesia). A strip of iron or steel does not
burn in air, but an iron wire or steel spring
may be easily burnt in oxygen. A much
larger piece of iron might naturally be burnt
if it only were convenient to heat it to the
required degree.27 The combustion of steel
or iron in oxygen is not accompanied by a
flame, but sparks of oxide fly in all directions
from the burning portions of the iron.28
In order to demonstrate by experiment
the combustion of hydrogen in oxygen, a gas-
conducting tube, bent so as to form a con-
..,.,,,, ,. , . . FIG. 30.— Mode of burning a steel
vemeiit jet, is led from the vessel evolving spring in oxygen.
hydrogen. The hydrogen is first set light
to in air, and then the gas-conducting tube is let down into a 'flask
containing oxygen. The combustion in oxygen will be similar to
that in air ; the flame remains pale, notwithstanding the fact that its
temperature rises considerably. It is instructive to remark that oxygen
may burn in hydrogen, just as hydrogen in oxygen. In order
to show the combustion of oxygen in hydrogen, a tube bent vertically
upwards and ending in a fine orifice is attached to the stop-cock of a
gas holder full of oxygen. Two wires, placed at such a distance from
-7 In order to burn a watch spring, a piece of tinder (or paper soaked in a solution of
nitre, and dried) is attached to one end. The tinder is lighted, and the spring is then
plunged into the oxygen. The burning tinder heats the end of the spring, the heated
part burns, and in so doing heats the further portions of the spring, which thus entirely
burns if enough oxygen is present.
28 The sparks of rust are produced by reason of the volume of the oxide of iron being
nearly twice that of the volume of the iron, and as the heat evolved is not sufficient to en-
tirely melt the oxide or the iron, the particles must be torn off and fly about. Similar
sparks are formed in the combustion of iron, in other cases also. We saw the combustion
of iron filings in the Introduction. In the welding of iron small iron splinters fly off in all
directions and burn in the air, as is seen from the fact that whilst flying through the air
they remain red hot, and also because, on cooling, they are seen to be no longer iron, but
a compound of it with oxygen. The same thing takes place when the hammer of a gun
strikes against the flint. Small scales of steel are heated by the friction, and glow and
burn in the air. The combustion of iron is still better seen by taking it as a very fine
powder, such as is obtained by the decomposition of certain of its compounds — for
instance, by heating Prussian blue, or by the reduction of its compounds with oxygen by
hydrogen ; when this fine powder is strewn in air, it burns by itself, even without being
previously heated (it forms a pyrophorus). This obviously depends on the fact that the
powder of iron presents a larger surface of contact with air than an equal weight in a
compact form.
each other as hi allow tin- passage of a constant series of sparks from a
Ividimkorii"s coil, are lixed in from of the oriiicc of the tube. This is
in order to ignite the oxvgen. which nia\' also he done bv udtach-
HIL; tinder round the ontire. and burning it. \\hen tlie wires are
arranged about the onl'.ee of the tube, and a series of sparks passes
! i» •; wren I IK 'in. 1 1 it MI an in \ ert ed ( because of the lightness of the hydro-
uvn i jar full of hydrogen is placed over the gas-conducting tube.
\\'hen the jarco\ers the orilice of the gas-conducting tube (and not
1 iff ore. as otherwise an explosion mi^ht take place ) the cock of the gaso-
meter is opened, and the oxvgen tlows into the hvdro^en and is set liidit
t«> by the sparks. The tlanie obtained is similar to that formed bv the
combustion of hydrogen in oxygen.'-"-' I'Yom this it is evident that tlie
tiaine is the localitv where the oxygen combines with the hydrogen,
then-fore a tlanie of burning oxygen can be obtained as well as a tlanie
of 1 lurninu' hvdrogen.
If. instead of hvdrogen. anv other combustible gas be taken -for
example, ordinary coal gas then the phenomenon of combustion will
be exactly the same, onlv a bright llame will be obtained, and the
products nf combustion will be different. However, as lighting gas
contains a considerable amount of free and combined hydrogen, it will.
also form a considerable (juantitv of water in its combustion.
If hvdrogen be mixed with o.xyuvn in the proportion in which thev
form water -i.e., if two volumes of hydrogen be taken for each
\olume of OXVLMMI then the mixture will he the same as that obtained
bv the decomposition of watt r bv a galvanic1 current detonating
• • | !•]• nicnl may l.c coinliictcd witlmut tin- \viiv-. if tin- liytlr..Lrrii In- li-lilrd in
lir. ,.| :, ryliiul.T. .MM! ,n I in- ,:iin,. !ini»' tin- rylin.l.T !>!• hrmi-'lit <.V<T tin- cud of a
>|. f 1,-ii in; i.\\-.-n. iiixl tin- i.llu-r with n -a-lit.ld.-r lull nf liyiln^i-n.
i! : . llh In (| )-, ,_cii i- li-litcil. .iinl a c ...... nut! la nip i: las-. ta|'ci-iii'_r
• • ' i . . jilai > 'i ''.< i tin' (-'irk. 'I'lii- liydr.i-i'ii nuil nin^ In Imi-n in-^iiic the
. • ,|i . . a! ! In • •• • |" •!!-•• n! tin1 n\\ jcn. It I In- ciirrcnl n| d\\;.'t'ii I"1 lln-n lit 1 !•• li\ litt !<•
; ,i . . Tl ;:
al . IHI!\ 1 ii- i ni Tea
I. id Ml li\di'i'ji , id !
and : can ea il\ I »< ,r.i\
IM tin- in^iitiicifiit siipi'lv nl' u.\\ LTi-ii. the IliiiiH-
i| I'.ir . \ , • I, i I 1 1 1 1 -I in 1 1 1 - . a 1 1 < I ! I U • 1 1 f. -a | »| '« 'a r> a t
,\\ i,f i..\\ -i-ii In- a.-_-ain im-n-iiM-il, the llaim- rr-
ll.iini- max In- mad.' ;<> a|>|u-ar at one nr the
di-i-n-a e nf tile elllTelil nl •_• a -, in list In- I>V
hi-ii Li- slin\\n liu\\ air Imnis in an at nn>>jihere
ai ill'- lam _dass is mil nl a -ja- eniiil)Ust ilile
OXYGEN AND ITS SALINE COMBINATIONS
167
an electric spark, because the spark heats the space through which it
passes, and acts consequently in a manner similar to ignition by means
of contact with an incandescent or burning substance. In fact, instead
of a spark a fine wire simply
may be taken, and an elec-
tric current passed through
it to bring it to a state of
incandescence ; in this case
there will be no sparks, but
the gases will inflame if the
wire be fine enough to be-
come red hot by the passage
of the current. Cavendish
made this experiment on the
ignition of detonating gas,
at the end of the last cen-
tury, in the apparatus shown
in fig. 31. Ignition by the
aid of the electric spark is
convenient, for the reason
that it may then be brought
about in a closed vessel,
and hence chemists still em-
ploy this method when it is FIG. 31.— Cavendish's apparatus for exploding detonatin
. . . gas. The bell jar standing in the bath is filled wit
required to ignite a mixture
of oxygen with a combus-
tible gas in a closed vessel.
For this purpose they now,
especially since Bunsen's
time,30 employ an eudiometer.
It consists of a thick glass tube graduated along its length in milli-
metres (for indicating the height of the mercury column), and
calibrated for a definite volume (weight of mercury). Two plati-
num wires are fused into the upper closed end of the tube, as
shown in fig. 32. They must be hermetically sealed into the tube,
so that there be no aperture left between them and the glass.31
50 Now, a great many other different forms of apparatus, sometimes designed for
special purposes, are employed in the laboratory for the investigation of gases. Detailed
descriptions of the methods of gas analysis, and of the apparatus employed, must be
looked for in works on analytical and applied chemistry.
31 In order to test this, the eudiometer is filled with mercury, and its open end
inverted into mercury. If there be the smallest orifice at the wires, the external air will
enter into the cylinder and the mercury will fall, although not rapidly if the orifice
be very fine.
a mixture of two volumes of hydrogen and one volume of
oxygen, and the thick glass vessel A is then screwed
into it. The air is first pumped out of this vessel, so
that when the stop-cock c is opened, it becomes filled
with detonating gas. The stop cock is then re-closed,
and the explosion produced by means of a spark from
a Leyden jar. After the explosion has taken place the
stop-cock is again opened, and the water rises into the
vessel A.
168
PRINCIPLES OF CHEMISTRY
By the aid of the eudiometer we may not only determine the volu-
metric composition of water,32 and the quantitative contents of oxygen
•I'!' 5- The eudiometer is used for determining the composition of combustible
gases. A detailed account of gas analysis would be out of place in this work
(see Note 30), but, as an example, we will give a short description of the deter-
mination of the composition of water by the eudiometer.
Pure and dry oxygen is first introduced into the eudiometer. When the
eudiometer and the gas in it acquire the temperature of the surrounding
atmosphere — which is recognised by the fact of the meniscus of the mercury
not altering its position during a long period of time — then the heights at
which the mercury stands in the eudiometer and in the bath are observed.
The difference (in millimetres) gives the height of the column of mercury in
the eudiometer. It must be reduced to the height at which the mercury
would stand at 0° and deducted from the atmospheric pressure, in order to
find the pressure under which the oxygen is measured (see Chap. I. Note 29).
The height of the mercury also shows the volume of the oxygen. The tem-
perature of the surrounding atmosphere and the height of the barometric
column must also be observed, in order to know the temperature of the oxy-
gen and the atmospheric pressure. When the volume of the oxygen has been
measured, pure and dry hydrogen is introduced into the eudiometer, and the
volume of the gases in the eudiometer again measured. They are then ex-
ploded. This is done by a Leyden jar, whose outer coating is connected by
a chain with one wire, so that a spark passes when the other wire, fused into
the eudiometer, is touched by the terminal of the jar. Or else an electrophorus
is used, or, better still, a Ruhmkorff's coil, which has the advantage of work-
ing equally well in damp or dry air, whilst a Ley Jen jar or electrical machine
does not act in damp weather. Further, it is necessary to close the lower
orifice of the eudiometer before the explosion (for this purpose the eudio-
meter, which is fixed in a stand, is firmly pressed down from above on to a piece
of india-rubber placed at the bottom of the bath), as otherwise the mercury
and gas would be thrown from the apparatus by the explosion. It must
also be remarked that to ensure complete combustion the proportion between
the volumes of oxygen and hydrogen must not exceed twelve volumes of
hydrogen to one volume of oxygen, or fifteen volumes of oxygen to one
volume of hydrogen, because no explosion will take place if one of the gases
be in great excess. It is best to take a mixture of one volume of hydrogen
with several volumes of oxygen. The combustion will then be complete. It is
FIG. 32.— evident that water is formed, and that the volume (or tension) is diminished,
Eudiometer. 8O thati on opening the end of the eudiometer the mercury will rise in it.
But the tension of the aqueous vapour is now added to the tension of the
gas remaining after the explosion. This must be taken into account (Chap. I. Note 1).
If there remain but little gas, the water which is formed will be sufficient for its satura-
tion with aqueous vapour. This may be learnt from the fact that drops of water are
visible on the sides of the eudiometer after the mercury has risen in it. If there be none,
a certain quantity of water must be introduced into the eudiometer. Then the number
of millimetres expressing the pressure of the vapour corresponding with the tempera-
ture of the experiment must be subtracted from the atmospheric pressure at which the
remaining gas is measured, otherwise the result will be inaccurate.
This is essentially the method of the determination of the composition of water which
was made for the first time by Gay-Lussac1 and Humboldt with sufficient accuracy.
Their determinations led them to the conclusion that water consists of two volumes of
hydrogen and one volume of oxygen. Every time they took a greater quantity of oxygen,
the gas remaining after the explosion was oxygen. When they took an excess of hydro-
gen, the remaining gas was hydrogen ; and when the oxygen and hydrogen were taken in
N AND ITS SALINE COMBINATIONS 169
in aiiyj;< but also make a number of experiments explaining the
phenomenon of combustion.
Thus, for example, it may be demonstrated, by the aid of the
eudiometer, that for the ignition of detonating gas a definite temperature
is required. If the temperature be below that required, combination
will not take place, but if at any spot within the tube it rises to the
temperature of inflammation, then combination will ensue at that spot,
and evolve enough heat for the ignition of the adjacent portions of the
detonating mixture. If to 1 volume of detonating gas there be added
10 volumes of oxygen, or 4 volumes of hydrogen, or 3 volumes of
carbonic anhydride, then we shall not obtain an explosion by passing
a spark through the diluted mixture. This depends on the fact that
the temperature falls with the dilution of the detonating gas by another
gas, because the heat evolved by the combination of the small quantity
of hydrogen and oxygen brought to incandescence by the spark is not
only transmitted to the water proceeding from the combination, but
also to the foreign substance mixed with the detonating gas.34 The
necessity of a definite temperature for the ignition of detonating gas is
also seen from the fact that pure detonating gas explodes in the presence
of a red-hot iron wire, or of charcoal so feebly incandescent as to be
hardly distinguishable by day light, but with a lower degree of in-
candescence there is not any explosion. It may also be brought about
by rapid compression, when, as is known, heat is evolved.3'"1 Experi-
ments made in the eudiometer showed that the ignition of detonating
gas takes place at a temperature between 450° and 5000.36
exactly the above proportion neither one nor the other remained. The composition of
water was thus definitely confirmed.
55 Concerning this application of the eudiometer, see the chapter on nitrogen.
31 Thus \ volume of carbonic oxide, an equal volume of marsh gas, two volumes of
hydrogen chloride or of ammonia, and six volumes of nitrogen or twelve volumes of air
added to one volume of detonating gas, prevent its explosion.
"" If the compression be brought about slowly, so that the heat evolved succeeds in
passing to the surrounding space, then the combination of the oxygen and hydrogen does
not take place, even when the mixture is compressed by 150 times ; for the gases are not
heated. If paper soaked with a solution of platinum (in aqua regia) and sal ammoniac
be burnt, then the ash obtained contains very finely-divided platinum, and in this form
it is best fitted for setting light to hydrogen and detonating gas. Platinum wire requires
to be heated, but platinum in so finely divided^ a state as it occurs in this ash inflames
hydrogen, even at — 203. Many other metals, such as palladium, iridium, and gold, act
with a slight rise of temperature, like platinum ; charcoal, like the majority of finely
divided substances, inflames detonating gas at 850°, but mercury, at its boiling point,
• I..,. s not inflame detonating gas. All data of this kind show that the explosion of
detonating gas presents one of the many cases of contact phenomena.
58 From the very beginning of the diffusion of the idea of dissociation, it might have
been imagined that reversible reactions of combination (the formation of Ho and O
belongs to this number) start at the same temperature as that at which dissociation
begins. And so it is in many cases, but not always, as may be seen from the facts (1) that
170
PRINCIPLES OF CHEMISTRY
The combination of hydrogen with oxygen is accompanied by the
evolution of a very considerable amount of heat ; according to
the determinations of Favre and Silbermann*1 1 part by weight of
hydrogen in forming water evolves 34462 units of heat. Many of the
most recent determinations are very near this figure, so that it may be
taken that in the formation of 18 parts of water (H2O) there are
evolved 69 major calories, or 69000 units of heat.38 If the specific heat
at 450-560 J, when detonating gas explodes, the density of aqueous vapour not only
does not vary (and it hardly varies at higher temperatures, probably because the amount
of the products of dissociation is small), but there are not, as far as is yet known, any
traces of dissociation ; (2) that under the influence of contact the temperature at which
combination takes place falls even to the ordinary temperature, when water and similar
compounds naturally are not dissociated and, judging from the data communicated by
D. P. Konovaloff (Introduction, Note 39) and others, it is impossible to escape the phe-
nomena of contact ; all vessels, whether of metal or glass, show the same influence as
spongy platinum although to a much less degree. The phenomena of contact, judging
from the mass of the data referring to it, must be especially sensitive in reactions which
are powerfully exothermal, and the explosion of detonating gas is of this kind.
57 The amount of heat evolved in the combustion of a known weight (for instance, 1
gram) of a given substance is determined by the rise in temperature of water, to which
the whole of the heat evolved in the combustion is transmitted. A calorimeter, for
example, that shown in fig. 33, is employed for this purpose. It consists of a thin (in
order that it may absorb less heat), polished (that it should transmit a minimum of heat)
metallic vessel, surrounded by down (c), or some other bad conductor of heat, and an outer
metallic vessel. This is necessary in order that the least possible amount of heat should
be lost from the vessels ; nevertheless, there is always a certain loss, whose magnitude
is determined by preliminary experiment (by taking
warm water, and determining its fall in temperature
after a definite period of time) as a correction for the
results of observations. The water to which the heat
of the burning substance is transmitted is poured
into the vessel. The stirrer g allows of all the layers
of water being brought to an equal temperature, ;m<l
the thermometer serves for the determination of the
temperature of the water. The heat evolved p;is>c>,
naturally, not to the water only, but to all the parts (A
the apparatus. The quantity of water corresponding
with the whole amount of those objects (the vessels,
tubes, &c.) to which the heat is transmitted is pre-
viously determined, and in this manner another most
important correction is made in the calorimetric deter-
minations. The combustion itself is carried on in the
vessel a. The ignited substance is introduced through
the tube at the top, which closes tightly. In fig. 3&
the apparatus is arranged for the combustion of a gas,
introduced by a tube. The oxygen required for the
combustion is led into a by the tube <?, and the pr<>-
PIG. 33. — Favre and Silbermann's calo- ducts of combustion either remain in the vessel a (if
"volve'd ifcombSln.^ "" ^ li(luid or solid), or escape by the tube/ into an n1MKua-
tus in which their quantity and properties can easily
be determined. Thus the heat evolved in combustion passes to the walls of the vessel a,
and to the gases which are formed in it, and these transmit it to the water of the
calorimeter.
58 This quantity of heat corresponds with the formation of liquid water at the ordinary
OXYGEN AND ITS SALINE COMBINATIONS 171
of aqueous vapour (0'4S) remained constant from the ordinary tempera-
ture to tJutf of /'-/tick the combustion of detonating gas takes place (but
temperature from detonating gas at the same temperature. If the water be as vapour
the heat evolved = 58 major calories; if asice = 70'4 major calories. A portion of this
heat is due to the fact that 1 vol. of hydrogen and £ vol. of oxygen give 1 vol. of aqueous
vapour — that is to say, contraction ensues — and this evolves heat. This quantity of heat
may be calculated, but it cannot be said how much is expended in the tearing apart of
the atoms of oxygen from each other, and therefore, strictly speaking, we do not know
the quantity of heat which is evolved in the combination of hydrogen with oxygen ;
although the number of units of heat evolved in the combustion of detonating gas is
accurately known.
The construction of the calorimeter and even the method of determination vary
considerably in different cases. The greatest number of calorimetric determinations were
made by Berthelot and Thomsen. They are given in their works Essai de mecanique
cJiiiniquc fonilea sur la thcrmucltimie, by M. Berthelot, 1879 (2 vols.), and thermo-
chcmische Untersnchu)it/en, by J. Thomsen, 1886 (4 vols.). The student must refer to
works on theoretical and physical chemistry for a description of the elements and methods
of thermochemistry, into the details of which it is impossible to enter in this work, all
the more so because, as has been shown of late, both the theoretical side of this subject
and its practical methods are still in an elementary state of development, and must be
subjected to improvement in many aspects before thermochemical study can be of that
enormous utility to chemical mechanics which was expected from it at the time of the
appearance of the first researches in its province. One of the originators of thermo-
chemistry was a member of the St. Petersburg Academy of Sciences, Hess. Since 1870
a mass of researches have appeared in this province of chemistry, especially in France
and Germany, after the leading works of the French Academician, Berthelot, and the
Copenhagen professor, Thomsen. Among Russians, Beketoff, Luginin, Cheltzoff, Chroust-
choff, and others are known by their thermo-chemical researches. The present epoch_of
thermochemistry, in the absence of a steadfast foundation (and the principle of maximum
work cannot be counted as such), must be considered rather as a collective one, wherein
the material of facts is amassed, and the first consequences arising from them are noticed.
In my opinion three essential circumstances prevent the possibility of extracting any
exact consequences, of importance to chemical mechanics, from the amassed and already
immense store of thermochemical data : (1) The majority of the determinations are con-
ducted in weak aqueous solutions, and, the heat of solution being known, are referred to
the substances in solution ; yet there is much (Chap. I.) which forces one to consider that
in solution water does not play the simple part of a diluting medium, but of itself
acts independently in a chemical sense on the substance dissolved. (2) The other chief
portion of thermochemical determinations is conducted by the ignition of substances
at high temperatures, and as yet we do not know the specific heat of many substances
at these temperatures. (3) Physical and mechanical changes (decrease of volume, diffu-
sion, and others) inevitably proceed side by side with chemical changes, and for the pre-
sent it is impossible, in a number of cases, to distinguish the thermal effect of the one
and the other kind of change. It is evident that the one kind of change (chemical) is essen-
tially inseparable and incomprehensible without the other (mechanical and physical) ; and
therefore it seems to me that thermochemical data will only acquire their true meaning
when the connection between the phenomena of both kinds (on the one hand chemical
and atomic, and on the other hand mechanical and molecular or between entire masses)
is explained more clearly and fully than is the case at present. As there is no
doubt that the simple mechanical contact, or the action of heat alone, on substances some-
times causes an evident and always a latent (incipient) chemical change — that is, a
different distribution or movement of the atoms in the molecules — it follows that purely
chemical phenomena are inseparable from physical and mechanical phenomena. This is
because the atomic relations forming the essence of the chemical relations of a substance
are not observable, and at present are incomprehensible, without the molecular relations
172 PEINCIPLES OF CHEMISTRY
most probably it increases), were the combustion concentrated at one
point39 (but it occurs as a flame), were there no loss from radiation and
heat conduction, and, chiefly, did dissociation not take place — that is, did
not a state of equilibrium between the hydrogen, oxygen, and water come
about — then it would be possible to calculate the temperature of tlie flame
of detonating gas. It would then be 100000.40 In reality it is very
much lower, but it is nevertheless higher than the temperature attained
in furnaces and flames, and reaches up to 2000°. The explosion of
detonating gas is explained by this high temperature, because the
aqueous vapour formed must occupy a volume at least 5 times greater
than that occupied by the detonating gas at the ordinary temperature.
Detonating gas emits a sound, not only as a consequence of the
commotion which occurs from the rapid expansion of the heated vapour,
but also because it is immediately followed by a cooling effect, the
conversion of the vapour into water, and a rapid contraction.41
forming the essence of the physical relations, and even without the relations of the entire
masses of molecules evincing themselves in purely mechanical relations, inasmuch as
an individual atom is something unreal and fantastic. A mechanical change may be
imagined without a physical change, and a physical without a chemical change (although
such a representation would be artificial), but it is impossible to imagine a chemical
change without a physical and mechanical one, for without them we should not perceive
it, and through them we attain it. There was a time when the province of physics
embraced the whole of chemistry and mechanics. In the present day they have been de-
veloped independently and been isolated from each other, but in the future a fresh conjunc-
tion is imminent, and is heralded by the laws of the conservation of matter and of energy.
59 The flame, or locality where the combustion of gases and vapours is accomplished,
is a complex phenomenon, ' an entire factory,' as Faraday says, and therefore we will
consider flame in some detail in one of the following notes.
40 If 34500 units of heat are evolved in the combustion of 1 part of hydrogen, and
this heat is transmitted to the resulting 9 parts by weight of aqueous vapour, then we
find that, taking the specific heat of the latter as 0'475, each unit of heat raises the
temperature of 1 part by weight of aqueous vapour 2'1° and 9 parts by weight (2'l-*-9)
0-23° ; hence the 34500 units of heat raise its temperature 7935°. If detonating gas is
converted into water in a closed space, then the aqueous vapour formed cannot expand,
and therefore, in calculating the temperature of combustion, the specific heat at a con-
stant volume must be taken into consideration ; it is 0'36 for aqueous vapour. This
figure gives a still higher temperature for the flame. In reality it is much lower, but the
results given by different observers are very contradictory (from 1700° to 2400°), the
discrepancies depending on the fact that flames of different sizes are cooled by radiation
to a different degree, but mainly on the fact that the methods and apparatus (pyro-
meters) for the determination of high temperatures, although they enable relative
changes of temperature to be judged, are of little use for determining their absolute
magnitude. By taking the temperature of the flame of detonating gas as 2000°, I give,
I think, the average of the most trustworthy determinations.
41 It is evident that not only hydrogen, but every other combustible gas, will give an
explosive mixture with oxygen. For this reason coal-gas mixed with air explodes
when the mixture is ignited. The pressure obtained in the explosions serves as the
motive power of gas engines. In this case advantage is taken, not only of the pressure
produced by the explosion, but also of that contraction which takes place after the
explosion. On this is based the construction of several motors, of which Lenoir's was
nXYCKN AND ITS SALINE COMBINATIONS
173
Mixtures of hydrogen and of various other gases with oxygen
are taken advantage of for obtaining high temperatures. By the
aid of such high temperatures metals like platinum may be melted
on a large scale, which cannot be
done in furnaces heated with char-
coal and fed by a current of air. The
burner, shown in fig. 34, is constructed
for the application of detonating gas
to the purpose. It consists of two
brass tubes, one fixed inside the other,
as shown in the drawing. The internal
central tube C C conducts oxygen, and
the outside, enveloping, tube E' E' con-
ducts hydrogen. Previous to their
egress the gases do not mix together,
so that there can be no explosion inside
the apparatus. When this burner is
in use C is connected with a gasholder
containing oxygen, and E with a gas
holder containing hydrogen (or some-
times C0al-£as). The flow of the FlG- 34-~ Safety burner for detonating gas,.
described in text.
gases can be easily regulated by
the stop-cocks O H. The flame is shortest and evolves the greatest
heat when the gases burning are in the proportion of 1 volume of
oxygen to 2 volumes of hydrogen. The degree of heat may be easily
judged from the fact that a thin platinum wire placed in the flame
easily melts. By placing the burner in the orifice of a hollow piece
of lime, a crucible A B is obtained in which platinum may be easily
melted, even in large quantities if the current of oxygen and
hydrogen be sufficiently great (Deville). The flame of detonating gas
may also be used for illuminating purposes. It is by itself very pale,
but owing to its high temperature it may serve for rendering infusible
objects incandescent, and at the very high temperature produced by the
detonating gas the incandescent substance gives a most intense light.
For this purpose lime, magnesia, or oxide of zirconium are used, as they
are not fusible at the very high temperature evolved by the detonating
gas. A small cylinder of lime placed in the flame of detonating gas,
if regulated to the required point, gives a very brilliant white
formerly, and Otto's is now, the best known. The explosion is usually produced by coal-
g.is and air, but of late the vapours of combustible liquids (kerosene, benzene) are
also being employed in place of gas (Chap. IX.). In Lenoir's engine a mixture of coal-
KJIS and air is ignited by means of sparks from a RuhmkorfF s coil, but in the most recent
marl lines the gases are ignited by the direct action of a gas jet.
1 , 1 PRINCIPLES OK CHEMISTRY
liu'ht. v. Inch \vas at one time proposed for illuminating lighthouses.
At present in the majority of cases electric light, osving to its constancy
and other advantages, has replaced it for this purpose. The light
produced bv lime in detonating gas is called the /JrHnimond fif/Jtf or
The above cases form examples of the combustion of (dements in
oxvgen, but exactly similar phenomena are observed in the conJiuxtion
of ••itiii j:ini //'/\. So, for instance, the solid, colourless, shmv substance,
naphthalene. (',,,! Fs, burns in air with a smoky (lame, whilst in oxygen
it continues to burn with a very brilliant llame. Alcohol, oil. and
other substances burn brilliantly in oxygen on conducting the oxygen
by a tube to the flame of lamps burning these substances. A high
temperature is thus evolved, which is sometimes taken advantage of
in chemical practice.
I n order to understand why combustion in oxvgen proceeds more
rapidlv. and is accompanied by a more1 intense heat etl'ect, than com-
bustion in air.it must be recollected that air is oxvgen diluted with
nitrogen, which does not support combustion, and therefore fewer par-
ticles of oxygen flow to the surface of a substance burning in air than
when burning in pure oxygen. The chief reason of the intensity of com-
bustion in oxygen is the high temperature acquired by the substance
burning in it. Let us consider as an example the combustion of sulphur
in air and in oxygen. If 1 gram of sulphur burns in air or oxvgen it
evolves in either case I'L'oO unitsof heat /.''., evolves sufficient heat for
heating i' •_'•"><> grams of water 1" ('. This heat is first of all transmitted
to th'- sulphurous anhydride, HO.,, formed by the combination of sulphur
with oxygen. In its combustion 1 gram of sulphur forms '2 grams
of sulphurous anhydride /'.'., the sulphur combines \\ith 1 gram of
o\vg('H. In order that 1 gram of sulphur should have access to 1 gram
of oxvgen in air. it is necessary that .">••! grams of nitrogen should
simultaneously reach the sulphur, because air contains seventy-seven
parts of nitrogen (by weight) per twenty-three parts of oxvgen. Thus
in the combustion of 1 gram of sulphur, the L'l'on units of heat are
t ransmitl od to '2 grains of sulphurous oxide and toat least .">• I grams of
nitrogen. As (Hr>f) units of heat are required to raise 1 gram of
sulphurous anhydride I ('., therefore L' grams require <)•.'}] units. So
also .">• 1 grains of nitrogen require •"»• I '/ O'L'll or O-S."> unitsof heat,
and therefore in order to raise both gases 1 ( '. n-.°, I 4- n-s:l or I'll
units of heat are required, lint as the combustion of the sulphur
evohcs '-'.I'-"'1' units of heat, therefore the gases miuht be heated (if
their sjH-citic heats remained constant) to ~ or 1D71 < '. That
OXYUKN AND ITS SALINE COM HI NATIONS 175
is, the maximum possible temperature of the flame of the sulphur
burning in air will be 1974° C. In the combustion of the sulphur
in oxygen the heat evolved (2250 units) can only pass to the '1 grains
of sulphurous anhydride, and therefore the highest possible tempera-
ture of the flame of the sulphur in oxygen will be =~- or 7L'">s .
O'ol
In the same manner it may be calculated that the temperature of char-
coal burning in air cannot exceed 2700°, while in oxygen it may attain
10100° C. For this reason the temperature in oxygen will always be
higher than in air, although (judging from what has been said re-
specting detonating gas) neither one nor the other temperature will
nearly approach the theoretical quantities.
Among the phenomena accompanying the combustion of certain
substances, the phenomenon of flame attracts attention. Sulphur,
phosphorus, sodium, magnesium, naphthalene, cvrc., burn like hydro-
gen with a flame, whilst in the combustion of other substances no
flame is observed, as, for instance, in the combustion of iron and
of charcoal. The appearance of flame depends on the capacity of the
combustible substance to yield gases or vapours at the temperature of
combustion. At the temperature of combustion, sulphur, phosphorus,
sodium, and naphthalene pass into vapour, whilst wood, alcohol, oil, &c.,
are decomposed into gaseous and vaporous substances. The com-
bustion of gases and vapours forms flames, and therefore a flame is
composed of the hot and incandescent gases and vapours produced by co/n-
bustion. It may be easily proved that the flames of such non-volatile
substances as wood contain volatile and combustible substances formed
from them, by placing a tube in the flame and drawing air from
it with an aspirator. Besides the products of combustion, com-
bustible gases and liquids, previously in the flame as vapours, collect in
the aspirator. For this experiment to succeed— -i.e., in order to really
extract combustible gases and vapours from the flame — it is necessary
that the suction tube should be placed inside the flame. The com-
bustible gases and vapours can only remain unburnt inside the flame,
for at the surface of the flame they come into contact with the oxy.vvn
of the air and burn.42 Flames are of different degrees of
42 Faraday proved this by a very convincing experiment on a candle flame. It one
arm of a bent glass tube be placed in a candle flame above the wick in tin1 dark pert ion
of the flame, then the products of the partial combustion of the stearin will pass up the
tube, condense in the other arm, and collect in a flask placed under it iti-. '•'•'» a- heavy
white fumes which burn when lighted. If the tube be raised into the upper lumi-
nous portion of the flame, then a dense black smoke which will not inflame aeeiimulates
in the flask. Lastly, if the tube be let down until it touches the wick, then little
but stearic acid condenses in the flask.
176
PRINCIPLES OF CHEMISTRY
brilliancy, according to whether solid incandescent particles occur in
the combustible gas or vapour, or not. Incandescent gases and
vapours emit but little light by themselves, and therefore give a paler
flame.43 If a flame does not
contain solid particles it is
transparent, pale, and emits
but little light.44 The flames
of burning alcohol, sulphur,
and hydrogen are of this kind.
A pale flame may be rendered
luminous by placing fine par-
ticles of solid matter in it.
Thus, if a very fine platinum
wire be placed in the pale
flame of burning alcohol— or,
better still, of hydrogen — then
the flame emits a bright light.
This is still better seen by sift-
ing the powder of an incom-
bustible substance, such as
fine sand, into the flame, or
by placing a bunch of asbestos
threads in it. Every brilliant
flame always contains some
kind of solid particles, or at least some very dense vapour. The flame
of sodium burning in oxygen has a brilliant yellow colour, from the
presence of particles of solid sodium oxide. The flame of magnesium
is brilliant from the fact that in burning it forms solid magnesia, which
becomes white hot, and similarly the brilliancy of the Drummond light
is due to the heat of the flame raising the solid non-volatile lime to a
state of incandescence. The flames of a candle, wood, and similar sub-
stances are brilliant, because they contain particles of charcoal or soot.
It is not the flame itself which is luminous, but the incandescent soot
it contains. These particles of charcoal which occur in flames may be
easily observed by introducing a cold object, like a knife, into the
Fw. 35.— Faraday's experiment for investigating the
different parts of a caudle flame.
43 All transparent substances which transmit light with great ease (that is, which
absorb but little light) are but little luminous when heated ; so also substances which
absorb but few heat rays, when heated transmit few rays of heat.
44 There is, however, no doubt but that very heavy dense vapours or gases under
pressure (according to the experiments of Frankland) are luminous when heated, be-
cause, as they become denser they approach a liquid or solid state. Thus detonating
gas when exploded under pressure is brightly luminous.
oXYGKN AND ITS SALINH COMBINATIONS
177
flame.1' The particles of charcoal burn at the outer surface of the
flame if the supply of air be sufficient, but if the supply of air that is,
of oxygen — be insufficient for their combustion the flame smokes, because
these unconsumed particles of charcoal are carried off by the current
of air.4(J
45 If hydrogen gas be passed through a volatile liquid hydrocarbon — for instance,
through benzene (the benzene maybe poured directly into the vessel in which hydrogen is
generated) — then its vapour burns with the hydrogen and gives a very bright flame,
because the resultant particles of carbon (soot) are powerfully ignited. Benzene, or
platinum gauze, introduced into a hydrogen flame may be employed for illuminating
purposes.
46 Inflames the separate parts may be distinguished with more or less distinctness.
That portion of the flame whither the combustible vapours or gases flow, is not
luminous because its temperature is still too low for the process of combustion to take
place in it. This is the space which in a candle surrounds the wick, or in a gas jet
is immediately above the orifice from which the gas escapes. In a candle the com-
bustible vapours and gases which are formed by the action of
heat on the melted tallow or stearin, rise in the wick, and
are heated by the high temperature of the flame. By the
action of the heat, the solid or liquid substance is here, as
in other cases, decomposed, forming products of dry dis-
tillation. These products occur in the central portion of the
flame of a candle. The air travels to the flame from the
outside, and is not able to intermix with the vapours and
gases in all parts of the flame ; consequently, in the outer
portion of the flame the amount of oxygen flowing to it
will be greater than in the interior portions of the flames.
But, owing to diffusion, the oxygen, naturally together with
nitrogen, flowing to the combustible substance penetrates
inside the flame, when the combustion takes place in
ordinary air. The combustible vapours and gases combine
with this oxygen, evolve a considerable amount of heat, and
bring about that state of red heat which is so necessary
both for keeping up the combustion and also for the uses
to which the flame is applied. Passing from the colder
envelope of air to the interior of the flame, to the source of
the combustible vapours (for instance, the wick), we evidently
first traverse layers of high temperature, and then
layers of lower and lower temperature, in which the com-
bustion is less complete, owing to the limited supply of
oxygen.
Thus, yet unburnt products of the decomposition of
organic substances occur in the interior of the flame. But flamJG^ie p^fon (? contains
there is always free hydrogen in the interior of the flame, even the vapours and products of
when oxygen is introduced there, or when a mixture of ^e^^he'combustionTias coni-
hydrogen and oxygen burns, because the temperature menced, and particles of carbon
evolved in the combustion of hydrogen or the carbon of are emitted : and in the pale
zone B the combustion is corn-
organic matter is so high that the products of combustion pieted.
are themselves partially decomposed — that is, dissociated —
at this temperature. Hence, in a flame a portion of the hydrogen and of the oxygen
which might combine with the combustible substances must always occur in a free
state. If a hydrocarbon burns, and we imagine that a portion of the hydrogen occurs in
a free state, then a portion of the carbon must also occur in the same form in
VOL. I. N
ITS
thi- '- oh-erved in reality in the coinliUstioii of various h\ drocai'hon-. ('harcoal, or
the soot of a common flame, proceeds from the di ciation ol ,.. janic -nh-tances con-
tained in the tlame. The majority oMiydrocarl.on-. e-pedallv those containing much
stance, naphthalene— hum even in oxyp-n. w;th -eparatioiiof soot. The
hums, hul tin- carhoii n I. . t.] lly so. It is this free
, , i \\ 1 1 id i causes the 1 ir ill iancy of the flat e. That tl • • of the flame contains
. .vhich is still ca] ' • 1 illowiiiLT experi-
t ; A portion of the ibises may In willidrax i 1-y an ' the central portion
e flame of carhonic oxide, wliicli is comhu-t ihle in air. For thi- purpo-e Deville
pa-sed water tlmm-h a metallic tuhe haviiiL' a ti ie lateral or:tice. which i- placed in the
flame. As the water parses alon- the tnhe the -a -e- oi the l!a me enter it . they are
•, mipled l.y . ylinders of water pas-ine' alon- the ml,,., and are carried off with it into
• [< for their investigation. It appears that all portion- of the flame obtained
l,v the cond'Ustion of a mixture of earl • tain a portion of this
ture -till unl.urnt. The re-earch.-s o! [Vville and i'.u i wed that in the
-ion of a mixture of hydi'op-u and of c • vp-n in a closed
space, complete comliustioii sometimes does iiof take place immediately. It two
volumes of hydro-en and one volume of oxyp-n he enclosed in a closed -pace, then mi
explosion the pressure doe- not attain thai ma-mtude which it would were there
lediate and c-om]ilete comhustion. It may he calculated that in this ca-e the
pressure should attain twenty-six atmospheres. In reality, it has heen shown l>y
•\ experiment that in the explosion ol hydro-en and oxy-jvn the pressure does
n ; exceed nine and a-lialt atnifis]>heres.
This may he explained l.y the fact that, in : ;,< • .!•• of the oxyp-n
does not all nl once comhine with the ci I • The amount of apis
l,urnt may even l.e determined from the pressure produced in i'- coinliiist ion, knmvin^
tin- heat evolved in it- comhustion and the specific heat of all the resultant and partici-
pating -uhstances. and hence the tempera! lire of condni-t ion. and therefore also the
pressure which may he evolved a- a eoiise<|Uence of that ri-e of temperature which pro-
ceed- from the evolution of heat. It appeal's 1 hat in t his <-a -e onlv one-third of the pises
ilie at the I e|n] icra t lire e\'i 'I Veil ill
tioiiof the remaining mas-, which i- capaMi of luiruin--:'. The admi\ture of carhonic
interferes in the -ame manner. Thi- shows thai e\er\ portion of a tlame nin-t contain
hydro-'eu. hydrocarhoiis. carl ionic anhydride, and \\ ater. < '•<<.••• , ijiii'iit ly. /'/ /x tin i'«x>-il>lr
' me. A ci itii n i in
ffe rent ] ' . In thi pace differ, •,,••!,, ompoiienl parts are
Vel\ nliji cted to c.niihiisl ion. ' - il under iln '.'.<\< nee of adjacenl
,,|,jec1-. and c, mhii-tion onlv end- v,ln re ti:,- llame , nd . I' the coinlin-t ion coidd he
,.Mi,c,-n1ra1ed at - -pot. then the temperature /. o d,| I,,- in,- parahlv hii/ln-r llian it is
,Ul,lel' t lie a el i la I ci re 1 1 In -t a lice . ||ellce 't I not to he \\ o|i(|i I'ed at that -moke and soot
| „.,.., 1 1 -e t I'on i -.. hat ha ln-.-n aid a 1 - e, e complete com I ill ' >1 take place in-tan-
OXYCKX AM) ITS SALINK COM HINATH >NS 17'.)
inconsiderably. This may either proceed from the fact that ih»-
reaction of the substance (for example, tin, mercury, lead at a high
temperature, or a mixture of pyrogallol with caustic potash at the
ordinary temperature) evolves but little heat, or that the hoat
evolved is transmitted to good conductors of heat, like metals, or that
the combination with oxygen takes place so slowly that the heat
evolved succeeds in passing to the surrounding objects. Combustion
is only a particular, intense, and evident case of combination with
oxygen. Respiration is also an act of combination with oxygen ;
it also serves, like combustion, for the development of heat by
those chemical processes which are its consequences (the trans-
formation of oxygen into carbonic anhydride). Lavoisier enun-
ciated this in the clear expression, ' respiration is slow combus-
tion.'
Reactions of slow combination of substances with oxygen are
termed oxidations. Combination of this kind (and also combustion)
often results in the formation of acid substances, and hence the
name oxygen (Sauerstoff). Combustion is only rapid oxidation.
Phosphorus, iron, and wine may be taken as examples of substances
which slowly oxidise in air at the ordinary temperature. If such a
substance be left in contact with a definite volume of air or oxygen, it
little by little absorbs the oxygen, as may be seen by the decrease in
volume of the gas. This slow oxidation is, as a rule, rarely accom-
panied by a sensible evolution of heat ; but an evolution of heat really
occurs, only it is not apparent to our senses, owing to the inconsider-
able rise of temperature which takes place ; this is owing to the
slow rate of the reaction and to the transmission of the heat formed as
radiant heat, <fcc. Thus, in the oxidation of wine and its transformation
into vinegar by the usual method of its preparation, the heat evolved
cannot be observed because it extends over whole weeks, but in the
so-called rapid process of the manufacture of vinegar, when a large
quantity of wine is comparatively rapidly oxidised, the evolution of
heat is quite apparent.
Such slow processes of oxidation are always taking place in nature
by the action of the atmosphere. Dead organisms and the substances
obtained from them — such as bodies of animals, wood, wool, grass, &c. —
temperature. If they vary (as Berthelot and Vieille affirm), the portion of a substance
which remains unburnt on explosion cannot be calculated from the pressure, and there-
fore the quantitative side of the subject should be considered as doubtful. But the quali-
tative side of the subject cannot be subject to doubt, because the dissociation of the
products of combustion at high temperatures is proved clearly by the most varied
experiments.
x •_'
180 PRINCIPLES OF CHEMISTRY
are especially subject to this action. They rot and putrefy — that is,
their solid matter is transformed into gases, under the influence of
moisture, and atmospheric oxygen, and often under the influence of
other organisms, such as moulds, worms, micro-organisms (bacteria), and
suchlike. These are processes of slow combustion, of slow combination
with oxygen. Everyone knows that manure rots and evolves heat,
that stacks of damp hay, damp flour, straw, &c., become heated and
are changed in the process.47 In all these transformations there are
formed the same chief products of combustion as are contained in
smoke ; the carbon gives carbonic anhydride, and the hydrogen
water. Hence these processes require oxygen just like combustion.
This is the reason why the entire prevention of access of air hinders
these transformations,48 and an increased supply of air accelerates them.
The mechanical treatment of arable lands by the plough, harrow, and
other similar means has not only the object of facilitating the spread
of roots in the ground, and of making the soil more permeable to water,
but it also serves to facilitate the access of the air to the component
parts of the soil ; as a consequence of which the organic remains of
soil rot — so to speak, breathe air and evolve carbonic anhydride.
One acre of good garden land in summer evolves more than six tons
of carbonic anhydride.
It is not only vegetable and animal substances which are subject ta
slow oxidation in the presence of water. The very metals are rusted
under these conditions. Copper very easily absorbs oxygen in the
presence of acids. Many metallic sulphides (for example, pyrites) are
very easily oxidised with access of air and moisture. Thus processes
of slow oxidation proceed throughout nature.
There are many elements which do not, under any circumstances,,
combine directly with gaseous oxygen ; nevertheless their compounds
with oxygen may be obtained. Platinum, gold, iridium, chlorine,
and iodine are examples of such elements. In this case recourse is
had to a so-called indirect method — i.e., the given substance is-
47 Cotton waste (it is used in factories for cleaning machines from lubricating oil)
soaked in oil and lying in heaps is self-combustible, being oxidised by the air.
48 "When it is desired to preserve a supply of vegetable and animal food, the access of
the oxygen of the atmosphere (and also of the germs of organisms borne in the air)
is often prevented. For this reason articles of food ai-e often kept in hermetically closed
vessels, from which the air is withdrawn ; vegetables are dried and soldered up while hot
in tin boxes ; sardines are immersed in oil, &c. The removal of water from substances is
also sometimes resorted to with the same object (the drying of hay, corn, fruits), as also
is saturation with substances which absorb oxygen (such as sulphurous anhydride),
which hinder the growth of organisms forming the first cause of putrefaction, as in
processes of smoking, embalming, and in the keeping of fishes and other animal sj
mens in spirit, &c.
(>XV(iKN AND ITS SALINE CCLMI51NATX >NS 181
combined with another element, and by a method of double decom-
position this element is replaced by oxygen, or a substance is taken
which easily evolves oxygen, and is brought into contact with the given
substance. The oxygen then acts at the moment of its evolution. If
the conditions are such that the substance to be oxidised is liberated
at the same moment, then oxidation proceeds with greater ease.
(The explanation of this phenomenon was given in the last chapter.)
It must be remarked that substances which do not directly combine
with oxygen, but form compounds with it by an indirect method, often
readily lose the oxygen which was absorbed by them by double decomposi-
tion or at the moment of its evolution. Such, for example, are the com-
pounds of oxygen with chlorine, nitrogen, and platinum, which evolve
oxygen on heating. They, like other substances which easily evolve
oxygen on heating, may serve as a means for obtaining oxygen, or for
oxidation. They, in the presence of substances which are capable of
combining with oxygen, are decomposed, give up their oxygen to them,
and may thus be themselves employed for indirect oxidation. In this
respect oxidising agents, or those compounds of oxygen which are em-
ployed in chemical and technical practice for transf erring oxygen to
other substances, are especially remarkable. The most important
among these is nitric acid or aquafortis — a substance rich in oxygen,
and capable of evolving it when heated, and which easily oxidises a great
number of substances. Thus nearly all metals and organic substances
containing carbon and hydrogen are more or less oxidised when heated
with nitric acid. If strong nitric acid be taken, and a piece of burning
charcoal be immersed in the acid, it continues to burn, the combustion
proceeding in this case at the expense of the oxygen contained in
the liquid nitric acid. Chromic acid acts like nitric acid ; alcohol
burns when mixed with it. Although the action is not so marked,
even water may oxidise with its oxygen. Sodium is not oxidised in
perfectly dry oxygen at the ordinary temperature, but it burns very
easily in water and aqueous vapour. Charcoal can burn in carbonic
anhydride — a product of combustion— forming carbonic oxide. Mag-
nesium burns in the same gas, separating carbon from it. Generally,
combined oxygen can pass from one compound to another.
The products of combustion or oxidation — and in general the definite
compounds of oxygen — are termed oxides. Some oxides are not capable
of combining with other oxides— or combine with only a few, and then
form unstable compounds with the evolution of very little heat ;
others, on the contrary, enter into combination with very many other
oxides, and in general have remarkable chemical energy. The oxides
incapable of combining with others, or only showing this quality in a
small degree, are termed t ndijj'' r< it' <>,i-'i<l<-s. Such a re the peroxides, of
Nvhich mention li;is before brcli made.
1 he class (it oxides capable of entering into mutual combination
we Nvill term sit/ hi'' <>.<•></>, •>•. Thev t'all into two chief i^roups at least,
a- regards t he mo-t extreme members. Tin- members of one group do not
combine with each other, l>ut combine with the members of the other
u;roup. As representative <»f one group niav lie taken the oxides of
the metals, magnesium, sodium, calcium. iVc. Representatives of the
othi-r uToup are the oxides formed by the non-metals, sulphur, phos-
phorus, earbon. If we take, for instance, the oxide of calcium or
lime, and bring it into contact with oxides of the second ^roup, there
ensues very readv combination. rJ"lius. for instance, if \\-e mix calcium
oxid'- \\~ith oxide of phosphorus, thev combine \\ith i^reat tacilitv. with
the evolution of much heat. If we pass the vapour of sulphuric an-
hvdride. obtained by the combination of sulphurous oxide with oxv^en,
over pieces of lime heated to redness, then the sulphuric anhydride is
absorbed by the lime, with the formation of a substance called
calcium sulphate. I he oxides of the first kind, which contain
metals, are termed imxir a. r, <!,.-< iii' Imws. Lime is a familiar example
of this class. The oxides of the second group, which are capable of
combining with the bases, are termed a ithi/<lri< '• x <>f ///•• <iri</s or <n'n1
a. i-iil, N. Sulphuric anhydride, S( ). , may l>e taken as a type of the
v;roup. It is foi'iued by the combination of sulphur with oxygen : by
the addition ot a fresh (juantitv of oxx'gen to the above-mentioned
sulphurous anhvdride, S( ).,. b\- passing it and oxvgen o\'er incandescent
sponu;v platinum. ( 'arbonic anliydride loften termed "carbonic acid,'
(.'O.,). [ihosphoric anliydride, sulphurous anhydride, are all acid oxides,
fur thev can combine \\iih such oxides as lime or calcium oxide,
magnesia or magnesium oxide, .MgO, soda or sodium oxide. Na.,<),
iV ' ' .
It a i;]\eii element form one basic oxide, it is termed the n.i-i<li : for
example, calcium oxide, magnesium OXlile. potassium oxide. Some
indill'ereiit oxid«-s ai'e also called 'oxides ' it' i he\ ha \ e not t lie projiert ies
of peroxides. ;ind ai 1 he same time do not si IONS' the properties of acid
anh vd rides tor mst a nee. carbonic oxide, ot which men t ion has already
been made. If an clou'-lit forms t NS'o basic oxides (or t NS o indlHerent
oxide- not haxin^ the characteristics of a peroxide) then that of the
lower degree of ox ida t ion i- ca lied a stilio.i'i<li that is. su box ides contain
Ies- ox Vgen than oxides. 'Mills. \s hen copper Is hea t ed to 1'ei I ness 111 a
furnai-e it increases in \vei^ht and absorbs oxvifen, until for '»•"> pails
of copper thel'e is absorbed not more than > pa rt - of ox \gen b\- NS'eigllt,
tormiiiL;' a red muss. NS'hich is suboxide ot copper : but if the roasting
OXYGEN AM) ITS SALINK Co.MIHNATIoNs 183
be prolonged, and tin- draught of air be increased, 63 parts of copper
absorb 16 parts of oxygen, and form black oxide of copper. Some-
times to distinguish between the degrees of oxidation a change of
suffix is made in the oxidised element — ic oxide naming the higher
degree of oxidation, and — ous oxide the lower degree. Thus ferrous
oxide and ferric oxide are the same as suboxide of iron and oxide of
iron. This nomenclature is convenient in some cases, but cannot
always be employed. If an element forms one anhydride only, then it
is named by an adjective formed from the name of the element made to
end in — ic and the word anhydride. When an element forms two
anhydrides, then the suffixes — ous and — ic are used to distinguish
them : — ous signifying less oxygen than — ic ; for example, sulphurous
and sulphuric anhydrides.49 When several oxides are formed from the
same element, the prefixes mon, di, tri, tetra are used, thus : chlorine
monoxide, chlorine dioxide, chlorine trioxide, and chlorine tetroxide
or chloric anhydride.
Chemical transformations of the oxides themselves are rarely
accomplished, and in the few cases where they are subject to such
changes a particularly important part is played by their combinations
with water. The majority of, if not all, basic and acid oxides combine
with water, either by a direct or an indirect method forming hydrates
— that is, such compounds as split up into water and an oxide of the
same kind only. We already know that many substances are cap-
able of combining with water. Oxides possess this property in the
highest degree. We have already seen examples of this (Chap. I.)
in the combination of lime, and of sulphuric and phosphoric anhydrides,
with water. Hence the results of such combination are basic and acid
hydrates. Acid hydrates are called acids, because they have an acid
49 It must be remarked that certain elements form oxides of all three kinds — i.e.,
indifferent, basic, and acid ; for example, manganese forms manganous oxide, manganic
oxide, peroxide of manganese, red oxide of manganese, and manganic anhydride, although
some of them are not known in a free state but only in combination. It is, then, always to be
remarked that the basic oxide contains less oxygen than the peroxides, and the peroxides
less than the acid anhydride. Thus they must be placed in the following general normal
order with respect to the amount of oxygen entering into their composition — (1) basic
oxides, suboxides, and oxides; (2) peroxides; (8) acid anhydrides. The majority of
elements, however, do not give all three kinds of oxides, some giving only one degree
of oxidation. It must further be remarked that there are oxides fonned by the combina-
tion of acid anhydrides with basic oxides, or, in general, of oxides with oxides. For
every oxide having a higher and a lower degree of oxidation, it might be said that the in-
termediate oxide was formed by the combination of the higher with the lower oxide. But this
is not true in all cases— for instance, when the oxide under consideration forms a whole
series of independent compounds — for oxides which are really formed by the combination
of two other oxides do not give such independent compounds, but in many >
decompose into the higher and lower oxides.
184 PRINCIPLES OF CHEMISTRY
taste when dissolved in water (or saliva, for then only can they act on
the palate). Vinegar, for example, has an acid taste because it contains
acetic acid dissolved in water. Sulphuric acid, of which we have made
mention many times, because it is the acid of the greatest importance
both in practical chemistry and for its technical applications, is really
a hydrate formed by the combination of sulphuric anhydride with
water. Besides their acid taste, dissolved acids or acid hydrates have
the property of changing to red the blue colour of certain vegetable
dyes. Of these dyes litmus is particularly remarkable and much used.
It is the blue substance extracted from certain lichens, and is used for
dyeing tissues blue ; it gives a blue infusion with water. This
infusion, on the addition of an acid, changes from blue to red.5n
Basic oxides, in combining with water, form hydrates, of which,
however, very few are soluble in water. Those which are soluble in
water have an alkaline taste like that of soap or of water in which ashes
have been boiled, and are called alkalis. Further, alkalis have the
50 Blotting or unsized paper, soaked in a solution of litmus, is usually employed for
detecting the presence of acids. This paper is cut into strips, and is called lest paper ;
when dipped into acid it immediately turns red. This is a most sensitive reaction, and
may be employed for testing for the least traces of acids. If 10000 parts by weight of water
be mixed with 1 part of sulphuric acid, the coloration is distinctly perceptible, and it is
quite distinguishable on the addition of ten times more water. Certain precautions
must, however, be taken in the preparation of such very sensitive litmus paper. Litmus
is sold in lumps. Take, say, 100 grams of it ; pound it, and add it to cold pure water in
a flask. Shake and decant the water. Kepeat this three times. This is done to wash
away easily-soluble impurities, especially alkalis. Transfer the washed litmus to a
flask, and pour in (!00 grams of water, heat, and allow the hot infusion to remain for
some hours in a warm place. Then filter, and divide the filtrate into two parts. Add a
few drops of nitric acid to one portion, so that a faint red tinge is obtained, and then
mix the two portions. Add spirit to the mixture, and keep it thus in a stoppered bottle
(it soon spoils if left open to the air). This infusion may be employed directly ; it reddens
in the presence of acids, and turns blue in the presence of alkalis. If evaporated, a
solid muss is obtained which is soluble in water, and may be kept unchanged for any
length of time. The test paper may be prepared as follows : — Take a strong infusion of
litmus, and soak blotting-paper with it ; dry it, and cut it into strips, and use it as test-
paper for acids. For the detection of alkalis, the paper must be soaked in a solution
of litmus just reddened by a few drops of acid ; if too much acid be taken, the paper will
not be sensitive. Such acids as sulphuric acid colour litmus, and especially its infusion,
a brick-red colour, whilst more feeble acids, such as carbonic, give a faint red-wine tinge.
Test-paper of a yellow colour is also employed ; it is dyed by an infusion of turmeric roots
in spirit. In alkalis it turns brown, but regains its original hue in acids. Many blue
and other vegetable colouring matters may be used for the detection of acids and alkalis ;
for example, infusions of cochineal, violets, log-wood, &c. Certain artificially-prepared
substances and dyes may also be employed. Thus rosolic acid, C2oH1(jO3, and
phenolphthale'm, C..>0H14O4, are colourless in an acid, and red in an alkaline, solution.
Cyanine is also colourless in the presence of acids, and gives a blue coloration with
alkalis. These are very sensitive tests. Their behaviour in respect to various acids,
alkalis and salts sometimes gives the means of distinguishing substances from each,
other.
OXYGES AM) ITS SALINE COMIMNATInNs IS;")
property of restoring the blue colour to litmus which has been reddened
by the action of acids. The hydrates of the oxides of sodium and
potassium, NaHO and KHO, are examples of basic hydrates easily
soluble in water. They are true alkalis, and are termed caustic, because
they act very powerfully on the skin of animals and plants. Thus
NaHO is called ' caustic ' soda.
Thus, the saline oxides are capable of combining together and with
water. Water itself is an oxide, and not an indifferent one, for it can,
as wre have seen, combine with basic and acid oxides ; it is a represen-
tative of a whole series of saline oxides, intermediate oxides, capable of
combining with both basic and acid oxides. There are many such
oxides, which, like water, combine with basic and acid anhydrides — for
instance, the oxides of aluminium and tin, &c. From this it may be
concluded that all oxides might be placed, in respect to their capacity
for combining with one another, in one uninterrupted series, at one
extremity of which would stand those oxides which do not combine
with the bases — that is, the alkalis— while at the other end would be
the acid oxides, and in the interval those oxides which combine with
one another and .with both the acid and basic oxides. The further
apart are the members of this series the more stable are the compounds
they form together, the more energetically do they act on each other,
the greater the quantity of heat evolved in their reaction, and the
clearer is their saline chemical character.
We said above that basic and acid oxides combine together, but
rarely react on each other ; this depends on the fact that the majority
of them are solids or gases — that is, they occur in the state least prone
to chemical reaction. The gaseo-elastic state is with difficulty destroyed,
because it necessitates overcoming the elasticity proper to the gaseous
particles. The solid state is characterised by the immobility of its
particles ; whilst chemical action requires contact, and hence a dis-
placement and mobility. If solid oxides be heated, and especially if
they be melted, then reaction proceeds with great ease. But such a
change of state rarely occurs in nature or in practice. In a few furnace
processes only is this the case. For example, in the manufacture of
glass, the oxides contained in it combine together in a molten state.
But when oxides combine with water, and especially when they form
hydrates soluble in water, then the mobility of their particles increases
to a considerable extent, and their reaction is greatly facilitated. Re-
action then takes place at the ordinary temperature — easily and rapidly ;
so that this kind of reaction belongs to the class of those which take
place with unusual facility, and are, therefore, very often taken advan-
tage of in practice, and also have been and are going on in nature at
186 PRINCIPLES OF CHEMISTRY
every step. We will now consider the reactions of oxides in the state
of hydrates, not losing sight of the fact that water is itself an oxide
with definite properties, and has. therefore, no little influence on the
course of those changes in which it takes part.
If we take a definite quantity of an acid, and add an infusion of
litmus to it, it turns red ; the addition of an alkaline solution does not
at once alter the red colour of the litmus, but on adding more and
more of the alkaline solution a point is reached when the red colour
changes to violet, and then the further addition of a fresh quantity of
the alkaline solution changes the colour to blue. This change of the
colour of the litmus is a consequence of the formation of a new com-
pound. This reaction is termed the saturation or neutralisation of
the acid by the base, or vice versa. The solution in which the acid
properties of the acid are saturated by the alkaline properties of the
base is termed a neutral solution. Such a solution, although derived
from the mixture of a base with an acid, does not, however, exhibit
either the acid or basic reaction on litmus, yet it preserves many other
signs of the acid and alkali. It is observed that in such a definite
admixture of an acid with an alkali, besides the change in the colour
of litmus, there is a heating effect — i.e., an evolution of heat — which is
alone sufficient to prove that there was chemical action. And, indeed,
if the resultant violet solution be evaporated, there separates out, not
the acid nor the alkali originally taken, but a substance which has
neither acid nor alkaline properties, but is usually solid and crystal-
line, having a saline appearance ; this is a salt in the chemical sense of
the word. Hence it is derived from the reaction of an acid on
an alkali, and through a definite relation between the acid and
alkali. The water here taken for solution plays no other part than
merely facilitating the progress of the reaction. This is seen from the
fact that the anhydrides of the acids are able to combine with basic
oxides, and give the same salts as do the acids with the alkalis or
hydrates. Hence, a salt is a compound of definite quantities of an
acid with an alkali. In the latter reaction, water is separated out if
the substance formed be the same as is produced by the combination of
anhydrous oxides together.51 Examples of the formation of salts from
acids and bases are easily observed, and are very often applied in
51 That water really is separated in the reaction of acid 011 alkaline hydrates, ni;iy In-
shown by taking some other intermediate hydrate — for instance, alumina — instead of
water. Thus, if a solution of alumina in sulphuric acid be taken, it will have, like the
acid, an acid reaction, and will therefore colour litmus red. If, on the other hand, a
solution of alumina in an alkali — for instance, potash — be taken, it will have an alkaline
reaction, and will turn red litmus blue. On adding the alkaline to the acid solution
until neither an alkaline nor an acid reaction is produced, a salt is formed, consisting of
OXYGEN AND ITS SALINE O OIlilN.XTK >NS 187
practice. If we take, for instance, insoluble nui^m-sium oxide, it i>
easily dissolved in sulphuric acid, and on evaporation ^m-s a saline
substance, bitter, like all the salts of magnesium, and familiar to
all under the name of Epsom salts, used as a purgative. If a solu-
tion of caustic soda — which is obtained, as we >a\\ , by the action of
water on sodium oxide — be poured into a flask in which charcoal has
been burnt ; or if carbonic anhydride, which is produced under so many
circumstances, be passed through a solution of caustic soda, then sodium
carbonate or soda, Na2C(X, is obtained, of which we have spoken several
times, and which is prepared on a large scale and often used in manu-
factures. This reaction is expressed by the equation, 2NaHO + CO2 =
Na2CO3 + H.)O. Thus, the various bases and acids form an innumer-
able number of different salts.52 Salts constitute an example of definite
chemical compounds which, both in the history and practice of science,
sulphuric anhydride and potassium oxide. In this, as in the reaction of hydrates, an
intermediate oxide is separated out — namely, alumina. Its separation will be very
evident in this case, as alumina is insoluble in water, whilst its compounds with the
acid and alkali, like the compound of an alkali with an acid — i.e.,& salt— are soluble
in water, and therefore on mixing the solutions of alumina in an acid and an alkali, it is
precipitated as a gelatinous hydrate.
5- The mutual interaction of hydrates, and their capacity of forming salts, may .be
taken advantage of for determining the character of such hydrates as are insoluble in
water. Let us imagine that a given hydrate, whose chemical character is unknown, is
insoluble in water. It is therefore impossible to test its reaction on litmus. It is then
mixed with water, and an acid — for instance, sulphuric acid — is added to the mixture. If
the hydrate taken be basic, reaction will take place, either directly or by the aid of
heat, with the formation of a salt. In certain cases, the resultant salt is soluble in
water, and this will at once show that combination has taken place between the
insoluble basic hydrate and the acid, with the formation of a soluble saline substance. In
those cases where the resultant salt is insoluble, still the water loses its acid reaction,
and therefore it may be ascertained, by the -addition of an acid, whether a given
hydrate has a basic character, like the hydrates of oxide of copper, lead, &c. If
the acid does not act on the given insoluble hydrate (at any temperature), then
it has not a basic character, and it should be tested as to whether it has an acid
character. This is done by taking an alkali, instead of the acid, and by observing
whether the unknown hydrate then dissolves, or whether the alkaline reaction dis-
appears. Thus it may be proved that hydrate of silica is acid, because it dissolves in
alkalis and not in acids. If it be a case of an insoluble intermediate hydrate, then it
will be observed to react on both the acid and alkali. Hydrate of alumina is an
instance in question, which is soluble both in caustic potash and in sulphuric acid.
But it must be remarked that intermediate oxides, in an anhydrous state, often
evince great resistance to the formation of saline compounds. Thus alumina or
aluminium oxide, in the anhydrous form in which it is met with in nature, and which
forms a crystalline substance, is insoluble in this form both in solutions of alkalis and
of acids. In order to convert it into a soluble form, it must be ground into a fine
powder and fused together with certain acid compounds, which are unchanged by
heat, such as acid potassium sulphate.
The degree of affinity or chemical energy proper to oxides and their hydrates is very
dissimilar ; some extreme members of the series have it to a great extent. When acting
on each other they evolve a large quantity of heat, and when acting on intermediate
hydrates they also evolve heat to a considerable degree, as we saw in the coinbi-
188
are most often cited a> confirming the conception of definite chemical
compounds. Indeed, all the indications of a definite chemical combina-
tion are clearlv seen in the formation and properties of >alts. Thus,
>alts are produced with a definite proportion of oxides, heat is evoked
in their formation/'3 and the character of the oxides and manv of their
physical properties are hidden in salts. Thus, when gaseous carbonic
anhydride combines with a base to form a solid >alt, the elasticity of
the u'as <|iiite disappears in its passage into the salt.'"'1
Judging tVom the above, a salt i^ a compound of basic and
nation ot lime and sulphunc anhydride with water. When extreme oxides combine they
lornistable salt-, which are ditticiiltlv decompo-ed. and often show characteristic proper-
ties. The compounds of the intermediate oxides with each other, or even with basic and
acid oxides, present a very different case. However much alumina we may dissolve
in sulphuric arid, we cannot saturate the acid properties of the sulphuric acid, the
resulting solution will always have an acid reaction. So aUo. whatever quantity of
alumina is dissolved in an alkali, the resultiiiLT solution will always present an alkaline
reaetii ui.
' In order to pve an idea of the quantity of heat evolved in the formation of salts,
I append a table of data tor rrry dilute arjncun* milittimix of acids and alkalis, accord-
ing to the determinations of Berthelot and Thonisen. The li-nres are pven in major
cal' -ries— t hat is. in thousands of units of heat. Hence. l'.» -'rams of sulphuric acid.
H SO., taken in a dilute aqueous solution, when mixed with such an amount of a weak
solution of call-tic soda, NallO. that a neutral salt i- form-d iwheii all the hvdn»_reii of
the acid i- replaced by the sodium), evolves l.">sUi) units of heat. A star signifies the
formation of an insoluble salt.
l .So4 II NO ,
Xallo . . ir.-s 1:5-7 M-o . l.vt; i:;-s
Kilo . . . ir.'T i:j-.s Fe() . }•!•:> ld'7 i?)
NH- . . . }{•:, \>i-:> '/.uO . . 11-7 i»-s
Ca<) . . . l.'.-t; ]:;•;) l°«-'-< • :''7 :'';l
'l'he-e (i '.Hires cjiinidt l>e considered as the heat of ueiit ralisat ion. l)ecause tin- water
here plays an important part. Thus, for instance, sulphuric acid and caustic soda in
dis-olviii'_' in water, evolve very much heat, and the result a ni -odium sulphate very little ;
con~c(| ui-iil I \ . the he;il e\iil\ed iii an a nliyilroiis -tale \\ill lie dit't'erent from (hat in a
1 1 vd rated state. Those acids wliich are not eiierj.fel ic in coiiiliiniiiir \\ it li the sal iiian-
titv of alkali- as i- reqiiired for the formation of normal -alts of sulphuric or nitric
acid- alwavs, howe\er. i:ive less heat. |-'or example, wit li caustic soda: carbonic acid
_;•.. Kr-J. liydi-ocyanic ii".i. 1 1 yd ro-eii sulphide :',".l. And as I'eelile liases (for example,
I-'e ( )- al-o evuh'c less heal tlian lliose uhich an- more powerful, so a certain ueiieral
correlation l.etwecn theniiochemical data and tlie conception of (lie measure of affinity
-hous itself here. ,i, in other cases i.srr Chap. II.. Note 7 1. which does not, however, -,'ive
• i on f..r juduriii^i »l the measure i.f the aftinit\ wliich l.inds the elements o) salts
h\ the heal of | he formation ,,| salts I,, dilute solutions. This is rendered especially
M the fact tli.it water is alile to decompose mam salts. ;<nd is -eparated in their
(>XV(iKN AM> ITS SALINE COMBINATIONS 189
acid oxides, or the result of the action of hydrates of these cl;i
on each other, with separation of water. But salts may be obtained
by other methods. Let us not forget that basic oxides are formed
by metals, and acid oxides often by non-metals. But metals and
non-metals are capable of combining together, and a salt is frequently
formed by the oxidation of such a compound. For example, iron very
easily combines with sulphur, forming iron sulphide (as we saw in the
Introduction) ; this in air, and especially moist air, absorbs oxygen,
with the formation of the same salt as may be obtained by the combina-
tion of the oxides of iron and sulphur, or of the hydrates of these
oxides. Hence, it cannot be said or supposed that a salt contains
the principles of the oxides, or that a salt must necessarily contain two
kinds of oxides in itself. The same conclusion may be arrived at by
investigating the different other methods of the formation of salts —
thus, for instance, many salts enter into double decomposition with the
metals, in which case the acting metal replaces that which originally
occurred in the salt. As we saw in the Introduction, iron, when placed
in a solution of copper sulphate, separates out the copper, and forms
an iron salt. Thus, the derivation of salts from oxides, is only
one of the methods of their preparation, there being many others,
and, therefore, it cannot be affirmed that a salt is simply the compound
of two oxides. We saw, for instance, that in sulphuric acid it was
possible to replace the hydrogen by zinc, and that by this means zinc
sulphate was formed ; so likewise the hydrogen in many other acids
may be replaced by zinc, iron, potassium, sodium, and a whole series of
similar metals, corresponding salts being obtained. The hydrogen in
the water of the acid, in this case, is exchanged for a metal, and a salt
is obtained from the hydrate. In this sense of a salt it may be said,
that a salt is an acid in which hydrogen is replaced by a metal. Such
a definition will be much more exact than that previously given, for it
refers directly to elements and not to their compounds with oxygen.
It shows that a salt and an acid are essentially compounds of the same
series, with the difference that the latter contains hydrogen and the
former a metal. Such a definition is still more exact than the first
definition of salts in respect to its referring likewise to those acids
which do not contain oxygen, and, as we shall afterwards learn, there
is a series of such acids. Such elements as chlorine and bromine form
ciating, evolves carbonic anhydride. The same gas, when dissolved in solutions of salts,
acts in one or the other manner (see Chap. II., Note 88). Here it is seen what a successive
series of relations exists between compounds of a different order, between sub-
stances of different degrees of stability. Were solutions distinctly separated from
chemical compounds, we should not be able to see those natural transitions which exist
in reality.
190 PRINCIPLES OF CHEMISTRY
compounds with hydrogen, in which the hydrogen may be replaced by
a metal forming substances which, in their reactions and external
characters, resemble the salts formed from oxides. Table salt, NaCl,
is an example of this. It may be obtained by the replacement of hydro-
gen in hydrochloric acid, HC1, by the metal sodium, just as sulphate
of sodium, NaaSO.,, may be obtained by the replacement of hydrogen
in sulphuric acid, H.2SO4, by sodium. The exterior appearance of the
resulting products, their neutral reaction, and even their saline taste,
show their mutual resemblance ; as the acid reaction, the property of
saturating bases, the capacity of exchanging their hydrogen for some
metal, and the acid taste, show the common properties belonging to
hydrochloric and sulphuric acids.
To the fundamental properties of salts yet another must be added —
namely, that they are more or less decomposed by the action of a galvanic
current. The results of this decomposition are very different, accord-
ing to whether the salt be taken in a fused or dissolved state. But
O
the decomposition may be so represented, that the metal appears at the
electro-negative pole (like hydrogen in the decomposition of water, or
its mixture with sulphuric acid), and the remaining parts of the salt
appear at the electro- positive pole (where the oxygen of water appears).
If, for instance, an electric current acts on an aqueous solution of sodium
sulphate, then the sodium appears at the negative pole, and oxygen
and the anhydride of sulphuric acid at the positive pole. But in the
solution itself the result is different, for sodium, as we know, decom-
poses water with evolution of hydrogen, forming caustic soda ; conse-
quently hydrogen will be evolved, and caustic soda appear at the
negative pole : while at the positive pole the sulphuric anhydride
immediately combines with water and forms sulphuric acid, and there-
fore oxygen will be evolved and sulphuric acid formed round this
pole.55 In other cases, when the metal separated is not able to decom-
pose water, it will be deposited in a free state. Thus, for example, in
the decomposition of copper sulphate, copper separates out at the
cathode, and oxygen and sulphuric acid appear at the anode, and
if a copper plate be attached to the positive pole, then the oxygen
evolved will oxidise the copper, and the oxide of copper will dissolve in
the sulphuric acid which is formed around this pole ; hence the copper
will be dissolved at the positive, and deposited at the negative, pole —
55 This kind of decomposition maybe easily observed by pouring a solution of sodium
sulphate in a U-shaped tube and inserting electrodes in both branches- If the solution
lae coloured with an infusion of litmus, it will easily be seen that it turns blue round the
electro-negative pole, owing to the formation of sodium hydroxide, and red at the
electro-positive pole, from the formation of sulphuric acid.
oXVdKN AND ITS SALINK ( '( >.M l',I NATI< >NS 191
that is, a transfer of copper from the positive to the negative pole
ensues. The galvanoplastic art (electrotyping) is based on this
principle/"1 Therefore the most radical and general properties of salts
(including also such salts as table salt, which contains no oxygen) may
be expressed by representing the salt as composed of a metal M and a
haloid X — that is, by expressing the salt by MX. In common table
salt the metal is sodium, and the haloid an elementary body, chlorine.
In sodium sulphate, Na2SO4, sodium is again the metal, but the
complex group, S04, is the haloid. In sulphate of copper, CuSO4, the
metal is copper, and the haloid the same as in the preceding salt.
Such a representation of salts expresses with great simplicity the
capacity of every salt to enter into saline double decompositions with
ntlicr salts ; consisting in the mutual replacement of the metals in the
salts. This exchange of their metals forms the fundamental property
of salts. If there be two salts with different metals and haloids, and
they be in solution or fusion, or any other manner, brought into con-
tact, then the metals of these salts will always partially or wholly
exchange places. If we designate one salt by MX, and the other by
NY, then we either partially or wholly obtain from them new salts,
MY and NX. Thus we saw in the Introduction, that on mixing
solutions of table salt, NaCl, and silver nitrate, AgNO3, a white
insoluble precipitate of silver chloride, AgCl, is formed, and a new salt,
sodium nitrate, NaNO3, is obtained in solution. If the metals of salts
exchange places in reactions of double decomposition, it is clear that
metals themselves, taken in a separate state, are able to act on salts, as
zinc evolves hydrogen from acids, and as iron separates copper from
copper sulphate. When, to what extent, and which metals displace each
other, and how the metals are distributed between the haloids, all this we
will discuss later on, guided by those reflections and deductions which
Berthollet introduced into the science at the beginning of this cen-
tury.
According to the above observations, an acid is nothing more than
a salt of hydrogen. Water itself may be looked on as a salt in which
56 In other cases the decomposition of salts by the electric current may be accom-
panied by much more complex results. Thus, when the metal of the salt is capable of a
higher degree of oxidation, such a higher oxide may be formed at the positive pole by
the oxygen which is evolved there. This takes place, for instance, in the decomposition
of salts of silver and manganese by the galvanic current, peroxides of these metals being
formed. If the metal separated at the negative pole acts on a salt occurring in the
solution, then it may do so at this pole, and in this manner the phenomena of the action
of a current on a salt are in many cases rendered remarkably complicated. But all the
phenomena as yet known may be expressed by the above law — that the current decom-
poses salts into metals, which appear at the negative pole, and into the remaining com-
ponent parts, which appear at the positive pole.
the hydrogen is combined with either oxygen or the aqueous radicle,
Oil : water will then he 11<>H, and alkalies or basic hydrates, MOIL
The group ()H. or the <-HJU.''<>/IS rftdiclt^ otherwise called Jti/dro.vyl, may
be looked on as a haloid like the chlorine in table salt, not only because
the element ( '1 and the group OH very often change places, and com-
bine with one and the same element, but also because free chlorine is
very similar in many respects and reactions to peroxide of hydrogen,
which is the same in composition as the aqueous radicle, as we shall after-
wards see. Alkalis and basic hydrates are also salts consisting of a
metal and hydroxyl— for instance, caustic soda, XaOH ; this is therefore
termed sodium Jiydroride. According to this view, nc.'nl xa/f* arc1 those
in which a portion only of the hydrogen is replaced bv a metal, and a
portion of the hydrogen of the acid remains. Thus sulphuric (H.,SO, |
acid with sodium not only gives the normal salt Xa^SO,. hut also an
acid salt, XallSO,. A //r/x/r wilt is one in which the metal is com-
bined not only with the haloids of acids, but also wit h the aqueous radicle
of basic hydrates -for example, bismuth gives not only a normal salt
of nitric acid, .l>i(X"( ):i)s, but also basic salts like I>i(< >H )L)(XO.<). As
basic and acid salts corresponding with the oxygen acids contain
hydrogen and oxygen, they are therefore able to part with these as
water and to give anhydro-salts, which it is evident will be equal to
compounds of normal salts with anhydrides of the acids or with bases.
Thus the above-mentioned acid sodium sulphate corresponds with
the anhydro-salt, Xa._,S._,O7, equal to L'XallSO,, less H2O. The loss
of water is here, and frequently in other cases, brought about bv
heat alone, and therefore such salts are frequently termed />yro-saffts
for instance, the preceding is sodium pyrosulphate (Xa.jS.^O-), or it may
be regarded as the normal salt X"a._,SO} -(- sulphuric anhydride, SOV
l)i,nl,Ii' salts are those which contain either two metals, 1\ A1(S( ),).,. or
two haloids/'7
••' The abo\e-enunciated generalisation of the conception of sa It s as compound- of
the metal- (simple, or compound like ammonium. N 1 1 , i, wit h the haloids i simple, like
impound, like cyanogen, CN. or the radicle of sulphuric acid, SO,), capable
'
lata respecting salts, was only formed little by little after a succession of
UK 1st \ a I'led
1
t
Salt- belong to the class of substances which have Ion- been known in \
hen-fore were ^udied in maiiv respects from very far back. At lirst, liowi
ii,ni\ artificial -all- during the latter half of the seventeenth eenturv. I'p 1
iractice. and
ler prepared
ii that time
•h \\ e have
1
aid icr's s-ilt
In iwed t heir
ction <,n \e -el able dye-., -till he c. ni t oi i n d e<l ii i a n y -a 1 1 s with acids i b\ the wa
V. we oll^llt,
be replaced
<>XY<JKN AND ITS sALINK COMBINATIONS 199
Inasmuch as oxygen compounds predominate in nature, it should
be expected, from what has been said above, that the occurrence of
salts, rather than of acids or bases, would be most frequent in nature,
for the latter on meeting, especially under the medium of the all-per-
by metals — that is, it is the hydrogen of an acid). Baume disputed Rouelle's opinion
concerning tin- subdivision of salts, contending that normal salts only are true salts, ami
that basic salts are simple mixtures of normal salts with bases and acid salts with acids,
considering that washing alone could remove the base or acid from them. Rouelle, in tin-
middle of the last century, however, rendered a great service to the study of salts and
the diffusion of knowledge respecting this class of compounds in his attractive lectures.
He, like the majority of the chemists of that period, did not employ the balance in his
researches, but satisfied himself with purely qualitative data. The first quantitative
researches on salts were carried on by Wenzel about this time. He was the director of
the Freiburg mines, in Saxony. Wenzel studied the double decomposition of salts, and
he observed that in the double decomposition of neutral salts a neutral salt was always
obtained. He proved, by a method of weighing, that this is due to the fact that the satura-
tion of a given quantity of a base requires such relative quantities of different acids as are
capable of saturating every other base. Having taken two neutral salts — for example,
sodium sulphate and calcium nitrate — let us mix their solutions together. Double
decomposition takes place, because the almost insoluble calcium sulphate is formed.
However much we might add of each of the salts, the neutral reaction will still be pre-
served, consequently the neutral character of the salts is not destroyed by the inter-
change of metals ; that is to say, that quantity of sulphuric acid which saturated the
sodium is sufficient for the saturation of the calcium, and that amount of nitric acid
which saturated the calcium is enough to saturate the sodium contained in combination
with sulphuric acid in sodium sulphate. Wenzel was even convinced that matter does
not disappear in nature, and on this principle he corrects, in his Doctrine of Affinity,
the results of his experiments when he remarked that he obtained less than he had origi-
nally taken. Although Wenzel deduced the law of the double decomposition of salts
quite correctly, he did not determine those quantities in which acids and bases act on
each other. This was done quite at the end of the last century by Richter. He deter-
mined the quantities by weight of the bases which saturate acids and of the acids which
saturate bases, and he obtained comparatively correct results, although his conclusions
were not correct, for he states that the quantity of a base saturating a given acid varies
in arithmetical progression, and the quantity of an acid saturating a given base in geo-
metrical progression. Richter studied the deposition of metals from their salts by other
metals, and observed that the neutral reaction of the solution is not destroyed by this
exchange. He also determined the quantities by weight of the metals replacing one
another in salts. He showed that copper displaces silver from its salts, and that zinc
displaces copper and a whole series of other metals. Those quantities of metals which
were capable of replacing one another were termed equivalents.
Richter's teaching found no followers, because, although he fully believed in the dis-
coveries of Lavoisier, yet he still held to the phlogistic reasonings which rendered his
expositions very obscure. The works of the Swedish savant Berzelius freed the facts
discovered by Wenzel and Richter from the obscurity of former conceptions, and led to
their being explained in accordance with Lavoisier's views, and in the sense of the law
of multiple proportions which had already been discovered by Dalton. On applying to
salts those conclusions which Berzelius arrived at by a whole series of researches of re-
markable accuracy, we are obliged to acknowledge the following law of equivalents —
oni' part by weight of hydrogen in an acid is replaced by the corrcsjHHtditiy i-<jnir<ilriif
irr'ujht of any metal ; and, therefore, when metals replace each other their weights are in
the same ratio as their equivalents. Thus, for instance, one part by weight of hydrogen
is replaced by 28 parts of sodium, 89 parts of potassium, 12 parts of magnesium, 20 parts
VOL. I. O
194 PRINCIPLES OF CHEMISTRY
vading water, form salts. And, indeed, salts are found everywhere
in nature. In animals and plants they occur, although in but small
of calcium, 28 parts of iron, 108 parts of silver, 33 parts of zinc, &c. ; and thei'efore, if zinc
replaces silver, then 33 parts of zinc will take the place of 108 parts of silver, or 33 parts
of zinc will be substituted by 23 parts of sodium, £c.
The doctrine of equivalents would be precise and simple did every metal only give
one oxide or one salt. It is rendered complicated from the fact that many metals form
several oxides, and consequently offer different equivalents in their different degrees of
oxidation. For example, there are oxides containing iron in which its equivalent is
28— this is in the salts formed by the suboxide ; and there is another series of salts
in which the equivalent of iron equals 18| — which contain less iron, and conse-
quently more oxygen, and correspond with a higher degree of oxidation — ferric
oxide. It is true that the former salts are easily formed by the direct action of
metallic iron on acids, and the latter only by a further oxidation of the compound
formed already ; but this is not always so. In the case of copper, mercury, and
tin, under different circumstances, there are formed salts which correspond with
different degrees of oxidation of these metals, and many metals have two equivalents
in their different salts — that is, in salts corresponding with the different degrees of
oxidation. Thus it is impossible to endow every metal with one definite equivalent
weight. Therefore the conception of equivalents, while playing an important part
from an historical point of view, appears, with a fuller study of chemistry, to be but an
incidental conception, subordinate to a higher one, with which we shall afterwards
become acquainted.
The fate of the theoretical views of chemistry was for a long time bound up with
the history of salts. The clearest representation of this subject dates back to
Lavoisier, and was very severely developed by Berzelius. This representation is called
the binary theory. All compounds, and especially salts, are represented as consisting
of two parts. Salts are represented as a compound of a basic oxide (a base) and an
acid (that is, an anhydride of an acid, then termed an acid), whilst hydrates are repre-
sented as compounds of anhydrous oxides with water. They employed such an expres-
sion not only to denote the most usual method of formation of these substances (which
would be quite true), but also to express that internal distribution of the elements by
which they proposed to explain all the properties of these substances. They supposed
copper sulphate to contain two most intimate component parts— copper oxide and
sulphuric anhydride. This is an hypothesis. It arose from the so-called electro-chemical
hypothesis, which supposed the two component parts to be held in mutual union,
because one component (the anhydride of the acid) has electro-negative properties, and
the other (the base in salts) electro-positive. Both parts are attracted together, like
substances having opposite electrical charges. But as the decomposition of salts in a
state of fusion by an electric current always gives a metal, therefore the representation
of the constitution and decomposition of salts, called the hydrogen theory of acids, is
more probable than that considering salts as made up of a base and an anhydride of
an acid. But the hydrogen theory of acids is also a binary hypothesis, and docs not
even contradict the electro-chemical hypothesis, but is rather a modification of it.
The binary theory dates from Kouelle and Lavoisier, the electro-chemical representation
was developed with great power by Berzelius, and the hydrogen theory of acids is due
to Davy and Liebig.
These hypothetical representations simplified and generalised the study of a com-
plicated subject, and gave support to arguments, but when salts were in question it
was equally convenient to follow one or the other of these hypotheses. But these
theories were brought to bear on all other substances, on all compound substances.
Those holding the binary and electro-chemical hypotheses searched for two anti-polar
component parts, and endeavoured to express the process of chemical reactions by electro-
chemical and similar differences. If zinc replaces hydrogen, they concluded that it is
OXYGEN AND ITS SALINE r< >M 111 NAT 1« >.\- 195
amount, because, as forming the last stage of chemical reaction, they
are capable of only a few chemical transformations, the energy of the
elements being evolved (passing into heat) both in the formation of
oxides and in their mutual combinations ; hence in salts there re-
mains but little energy. Organisms are bodies in which a series of
uninterrupted, varied, and active chemical transformations proceed,
whilst salts, which only enter into double decompositions between
each other, are incapable of such changes. But organisms always
contain salts. Thus, for instance, bones contain calcium phosphate,
the juice of grapes, potassium tartrate (cream of tartar), certain
lichens, calcium oxalate, and the shells of mollusca, calcium car-
bonate, &c. As regards water and soil, portions of the earth in
which the chemical processes are less active, they are full of salts.
Thus the waters of the oceans, and all others (Chap. I.), abound in
salts, and in the soil, in the rocks of the earth's crust, in the up-
heaved lavas, and in the falling meteorites the salts of silicic acid, and
more electro-positive than hydrogen, whilst they forgot that hydrogen may, under different
circumstances, displace zinc — for instance, at a red heat. Chlorine and oxygen were con-
sidered as being of opposite polarity to hydrogen because they easily combine with it, whilst
one and the other are capable of replacing hydrogen, and, what is very characteristic, in
the replacement of hydrogen by chlorine in carbon compounds, not only does the
chemical character often remain unaltered, but even the external form remains un-
changed, as Laurent and Dumas demonstrated. These considerations undermine the
binary theory, and especially the electro-chemical system. An explanation of known
reactions then began to be sought for not in the difference of the polarity of the
different substances, but in the joint influences of all the elements on the properties of
the compound formed. This is the reverse of the preceding hypotheses.
This reversal was not, however, limited to the destruction of the tottering founda-
tions of the preceding theory ; it projected a new doctrine, and laid the foundation for
the whole contemporary direction of our science. This doctrine may be termed the
unitary theory — that is, it is such as strictly acknowledges the joint influences of the ele-
ments in a compound substance, denies the existence of separate and contrary components
in them, regards copper sulphate, for instance, as a strictly definite compound of copper,
sulphur, and oxygen ; then seeks for compounds which are analogous in their properties,
and, placing them side by side, endeavours to express the influence of each element on
the united properties of its compound. In the majority of cases it arrives at systems of
consideration similar to those which are obtained by the above-mentioned hypotheses
but in certain special cases the conclusions of the unitary theory are in entire opposition
to the binary theory and its consequences. Cases of this kind are most often met with
in the consideration of compounds of a more complex nature than salts, especially
organic compounds containing hydrogen. But it is not in this revolution from an
artificial to a natural system, important as it is, that the chief service and strength of
the unitary doctrine lies. By a simple review of the vast store of data regarding the
reactions of typical substances, it succeeded from its first appearance in establishing a
new and important law, it introduced a new conception into science — namely, the
conception of molecules, with which we shall soon become acquainted. The deduction
of the law and of the conception of molecules has been verified by facts in a number of
cases, and was the cause of the majority of chemists of our times deserting the binary
theory and accepting the unitary theory, which forms the basis of the present work.
Laurent and Gerhardt must be looked on as the propagators of this doctrine.
o 2
: often form mountain chainsand wliole t hickn----e- of
-T rat a. t he-e consi.-t in n" of calcium earhonate, ( 'a< '* > .
I is \\ . • have -een o\v;_feii in a free -tate and in various compounds
of dill'crent decree- of stahilitv, from, the un-tahle salts, like I'.ert hollet '-
-•dt and nitre, to the most -laMe silicon compound-, -udi as exist in
granite. \\"e -,aw an entirely -imilar gradation of -lability in the com
Is of \\ater and of h vdro^en. In all it s aspect s oxygon, as an
element, a- a -uK-tance. remain- the same in it-elf in the nio-t varied
cliemical -tate-. just a- a -uhstance mav appear in dill'erent physical
1 a_;'u i'ei;at e ) -tales, l»ut our notion of the immense varietv of the
chemical >tates in which oxygen can occtii1 would nor lie completely
underst 1 if we did not make ourselves acquainted with it in the
Toi-m in \\hich it occur- in ozone and peroxide ot hydrri^en. In the-e
it i- nio-t active, its eiier^v seem- to ha\c inci'ea-ed. 'Then the fre-h
a-jiect- of chemical correlation-, ,-md the \ariet\- of the form- in which
in uter can appear, -land out (dearly. \Ve will therefore consider these
' wo -uhstances some\\-]iat in detail.
197
CHAPTER IV
OZONE AND HYDROGEN PEROXIDE. DALTON's LAW
VAN-MARUM, during the last century, observed that oxygen in a glass
tube, when subjected to the action of a series of electric sparks, acquired
a peculiar smell and the property of combining with mercury at the
ordinary temperature. This was afterwards confirmed by a number of
fresh experiments. Even in the simple revolution of an electrical
machine, when electricity diffuses into the air or passes through it, the
peculiar and characteristic smell proper to ozone, proceeding from
the action of the electricity on the oxygen of the atmosphere, is
recognised. In 1840 Prof. Schonbein, of Basle, turned his attention
to this odoriferous substance, and showed that it is also formed,
with the oxygen evolved at the positive pole, in the decomposition of
water by the action of a galvanic current ; in the oxidation of phos-
phorus in damp air, and also in the oxidation of a number of
substances, in consequence of which it is found in the atmosphere,
although it is distinguished for its instability and capacity for oxidis-
ing other substances. The characteristic smell of this substance (which
is always mixed with unaltered oxygen) gave it its name, from the Greek
o£w, ' to emit an odour.' Schonbein pointed out the characteristic pro-
perties of ozone, and especially its power of oxidising many substances,
even silver, acting like oxygen, but with this difference — that there are
a number of substances on which oxygen does not act at the ordinary
temperature, whilst ozone does so very energetically. It will be
enough to point out, for instance, that it oxidises silver, mercury,
charcoal, and iron with great energy at the ordinary temperature. It
might be thought that ozone was some new substance, simple or com-
pound, as it was at first supposed to be ; but careful observations
made in this direction have long led to the conclusion that ozone is
nothing but oxygen altered in its properties. This is most strikingly
proved by the complete transformation of oxygen containing ozone into
ordinary oxygen when it is passed through a tube heated to 250°.
Further, at a low temperature pure oxygen gives ozone when electric
IDS PKJNClPLKs ol- CHEMISTKY
-park- are passed through it ( Marii^nac and I >e l;i Rive). Hence it is
proved, by a method fur its preparation from oxygen and by a method of
its transformal inn into ox v^'en (synthesis and analysis), that oxone is tliat
same oxygen with which we art- already acquainted, nnly endowed with
particular properties and in a particular state. However, l»v whatever
method it IK' obtained, the anmiint of it contained in the oxygen is
inconsiderable, general Iv only a few fractions of a per cent., rarelv
'2 percent., and only under verv propitious circumstances as much as
l'<> per cent. The reason of this must be looked for first in the fact
that "•_"/" lit it* foi-inatioii from o.i'ij<i<'n dltxorb* If-nt. If any substance
be burnt in a calorimeter at the expense of o/onised oxygen, then more
heat is evolved than when it is burnt in ordinary oxygen, and Berthelot
showed that this ditl'erence is very lar^e namely, L'Uo'OU heat units
correspond with every forty-eiuht parts by weight of oxone. This
Minifies that the transformation of fort y-eii^ht parts of oxvgen into
nxone is accompanied by the absorption of this quantity of heat, and
that i In- reverse process evolves this quantity of heat. Therefore the
passage of oxone into oxygen should take place easily (as an exother-
mal reaction), like combustion : and this is pmved by the fact that at
•_'."»(> oxone cnlirelv disappears, foi-minn' oxyuvn. Anv rise of tem})era-
ture may thus brini;- about the breaking u]> of oxone. and as a rise of
temperature take.-j jilace in th(> action of an electrical discharge,
therefor** there are in an electric discharge the conditions both for the
preparation of oxone and for its destruction. Hence it is clear that
the transformation of oxygen into oxone, <>* >> /'ft't'i'siti/f i'>'<i<'t ton,
has a limit when a state of equilibrium rs arrived at between the
products of the i \\ ( i opposite reactions, that the phenomena of this
transformation accord \\ith t he phenomena of ili.<ot<n*mti<ni, and that a
fall of temperature should aid the format ion of a l;irg<- quantity of
oxone.1 Furiher. it is evident, from what ha- l»cen said, that the best
way of preparing oxone is not by electric sparks.- which raise the
p., nr]ii ii.n. rli..lm-,.,] l,y in-' a-. t,,r ku-k II- 1-7- MnHitritr Srii-ntijiijui-\l>\
, |l,i : I \I |f, |-1 l.ssil . wlui-ll >h..\V(..i lllilt tllr passil.UV .-I ;l -llclll
:;n i|, t., f.ii i • IH.-I .,] all i,, i ln< dclcrniinal .HI- nt' < 'li;i|>|»ui- and Haute
. ] --H . •/, |,M IMIIIK! lli. i' .il ii ! i -in] I. •]•..! nr.' i.i' -li.*. :i -liflil ili-rliar-T CdllVt-rtcd -Jl I p.c.
liil-1 .it 'JH i! \\.i- - iui|".--.il.lr t....l.la:n IIPHV 1 h. in 1- )'.<•.. an. hit lull
• \ ., , , , • , , n-ii i • . in. i\ In- (.lit; I I'itlirr l>\ aii ..rdiuarv i-lcrlrii-al inacliiiu-.
' ' Unit
OZONE AND HYDROGEN PEROXIDE — 1) ALTON'S LAW 199
temperature, but by the employment of a continual discharge or
flow of electricity — that is, to transform the oxygen by the action
of a silent discharge.3 For this reason all ozomsers (which are of
most varied construction), or forms of apparatus for the preparation of
ozone from oxygen (or air) by the action olf electricity, now usually
consist of conductors (sheets of metal — for instance, tinfoil — or a solution
of sulphuric acid with chromic acid, &c.) separated by thin glass
surfaces placed at short distances from each other, and between which
FIG. 37.— Siemens' apparatus for preparing ozone by means of a silent discharge.
the oxygen or air to be ozonised is introduced and subjected to the
action of a silent discharge.4 Thus in Siemens' apparatus (fig. 37) the
5 A silent discharge is such a combination of opposite statical (potential) electricities
as takes place (generally between large surfaces) regularly, without sparks, slowly, and
quietly (as in the dispersion of electricity). The discharge is only luminous in the dark ;
there is no observable rise of temperature, and therefore a larger amount of ozone is
formed. But, nevertheless, on continuing the passage of a silent discharge through
ozone it is destroyed. For the action to be observable a large surface is necessary, and
consequently a powerful source of electrical potential. For this reason the silent dis-
charge is best produced by a Ruhmkorff coil, as the most handy means of obtaining a
considerable potential of statical electricity with the employment of the comparatively
feeble current of a galvanic battery.
4 V.Sabo'n (ijijifirattm was one of the first constructed for ozonising oxygen bymeanB
•of a silent discharge (and it is still one of the best). It is composed of a number (twenty
and more) of long, thin capillary glass tubes closed at one end. A platinum wir-
tending along their whole length, is introduced into the other end of each tube, and this
end is then fused up round the wire, the end of which protrudes outside the tube.
The protruding ends of the wires are arranged alternately in two sides in such a manner
that on one side there are ten closed ends and ten wires. A bunch of such tubes (forty
should make a bunch of not more than 1 c.m. diameter) is placed in a glass tube, and
the ends of the wires are connected into two conductors, and are fused to the ends of the
surrounding tube. The discharge of a Ruhmkorff coil is passed through these cuds of
the wives, and the dry air or oxygen to be ozonised is passed through the tube. If
oxygen be passed through, ozone is obtained in large quantities, and free' from oxides of
200 PKINCIPLES OF CHEMISTRY
exterior of the tube a and the interior of the tube b c are coated with
tinfoil and connected with the poles of a source of electricity (with the
terminals of a Ruhmkorff's coil). A silent discharge passes through the
thin walls of the glass cylinders a and b c over all their surfaces, and
consequently, if oxygen be passed through the apparatus by the tube d,
fused into the side of «, it will be ozonised in the annular space between
a and b c. The ozonised oxygen escapes by the tube e, and may be
introduced into any other apparatus.5
The properties of ozone obtained by such a method'' distinguish it
in many respects from oxygen. Ozone very rapidly decolorises indigo,
litmus, and many other dyes by oxidising them. Silver is oxidised by
it at the ordinary temperature, whilst oxygen is not able to oxidise
silver even at high temperatures ; a bright silver plate rapidly turns
nitrogen, which are partially formed when air is acted on. It is remarked that at low
temperatures ozone is formed in large quantities. As ozone is acted on by corks and
india-rubber, the apparatus should be made entirely of glass. With a powerful Ruhmkorff
coil and forty tubes the ozonation is so powerful that the gas, when passed through a,
solution of iodide of potassium, not only sets the iodine free, but even oxidises it into
potassium iodate, so that in five minutes the gas-conducting tube is choked up with
crystals of the insoluble iodate.
5 In order to connect the ozoniser with any other apparatus it is impossible to make
use of india-rubber, mercury, or cements, etc., because they are themselves acted on by,
and act on, ozone. All connections must, as was first proposed by Brodie, be hermetically
closed by sulphuric acid, which is not acted on by ozone. Thus, a cork is passed over
the vertical end of a tube, over which a wide tube passes so that the end of the first tube
protrudes aboA'e the cork ; mercury is first poured over the cork (to prevent its being
acted on by the sulphuric acid), and then sulphuric acid is poured over the mercury.
The protruding end of the first tube is covered by the lower end of a third tube immersed
in the sulphuric acid.
6 The above-described method is the only one which has been well investigated. The
admixture of nitrogen, or even of hydrogen, and especially of silicon fluoride, appears to
aid the formation and preservation of ozone. Amongst other methods for preparing
ozone we may mention the following : — 1. In the action of oxygen on phosphorus at the
ordinary temperature a portion of the oxygen is converted into ozone. At the ordinary
temperature a stick of phosphorus, partially immersed in water and partially in air in a
large glass vessel, causes the air to acquire the odour of ozone. It must further be
remarked that if the air be left for long in contact with the phosphorus, or without the
presence of water, the ozone formed is destroyed by the phosphorus. 2. By the action
of sulphuric acid on peroxide of barium. If the latter be covered with strong sulphuric
acid (the acid, if diluted with only one-tenth of water, does not give ozone), then at a low
temperature the oxygen evolved contains ozone, and in much greater quantities than
that in which ozone is obtained by the action of electric sparks or phosphorus. 3. Ozone
may also be obtained by decomposing strong sulphuric- acid by potassium inanimate,
especially with the addition of barium peroxide. Gorup-Besanez stated (but it requires
confirmation) that ozone is formed in the slow evaporation of large quantities of water.
In the near proximity of salt-gardens (salterns) the atmosphere is considerably richer in
ozone than in the surrounding neighbourhood. In connection with this is the fact that
the air of the sea-shore is rich in ozone. Ozone is also stated to be formed in the
ordinary process of the respiration of plants. This is, however, denied by many to be
the case.
OZONE AM) HYDROGEN PEROXIDE— DAI/TON'S LAW 201
black (from oxidation) in. ozonised oxygen. It is rapidly absorbed by
mercury, forming oxide ; it transforms the lower oxides into higher — for
instance, sulphurous anhydride into sulphuric, nitrous oxide into
nitric, arsenious anhydride (As.2O3) into arsenic anhydride (As.205) <fcc. 7
But what is especially characteristic in ozone is the decomposing action
it exerts on potassium iodide. Oxygen does not act on it, but ozone
passed into a solution of potassium iodide liberates iodine, whilst the
potassium is obtained as caustic potash, which remains in solution,
2KI + H2O + 0=2KHO-|-l2. As the presence of minute traces of
free iodine may be discovered by means of starch paste, with which it
forms a very dark blue coloured substance, a mixture of potassium
iodide with starch paste will detect the presence of very small traces of
ozone.8 Ozone is destroyed or converted into ordinary oxygen not
only by heat, but also by long keeping, especially in the presence of
alkalis, peroxide of manganese, chlorine, tkc.
Hence ozone, although it has the same composition as oxygen, differs
7 Ozone takes up the hydrogen from hydrochloric acid ; the chlorine is set free, and
can dissolve gold. Chromium and iodine are directly oxidised by ozone, but not by oxygen,
and so also with a number of other substances. Ammonia, NH5, is oxidised by ozone into
ammonium nitrite (and nitrate), 2NH5 + O3 = NH4NO^ + H2O, and therefore a drop of
ammonia, on falling into the gas, gives a thick cloud of the salts formed. Ozone converts
lead oxide into peroxide, and suboxide of thallium (which is colourless) into oxide (which
is brown), so that this reaction is made use of for discovering the presence of ozone.
Lead sulphide, PbS, is converted into sulphate, PbSO4, by ozone. A neutral solution of
manganese sulphate gives a precipitate of manganese peroxide, and an acid solution may
be oxidised into permanganic acid, HMnO4. With respect to the oxidising action of ozone
on organic substances, it may be mentioned that with ether, C4H10O, ozone gives ethyl
peroxide, which is capable of decomposing with explosion (according to Berthelot), and is
decomposed by water into alcohol, 2C.>HeO, and hydrogen peroxide, EUO.j.
8 This reaction is the one usually made use of for detecting the presence of ozone.
In the majority of cases paper is soaked in solutions of potassium iodide and starch.
Such ozonometrical or iodised starch-paper when damp turns blue in the presence of ozone,
and the tint obtained varies considerably, according to the length of tune it is exposed and
to the amount of ozone present. The amount of ozone in a given gas may even to a
certain degree be judged by the shade of colour acquired by the paper, if preliminary
tests be made.
Test-paper for ozone is prepared in the following manner: — One gram of neutral
potassium iodide is dissolved in 100 grams of distilled water; 10 grams of starch are
then shaken up in the solution, and the mixture is boiled until the starch is converted
into a jelly. This jelly is then smeared over blotting-paper and left to dry. The colour
of iodised staivh-paper is changed not only by the action of ozone, but of many other
oxidisers ; for example, by the oxides of nitrogen and hydrogen peroxide. Houzeau pro-
posed soaking common litmus-paper with a solution of potassium iodide, which in the
presence of iodine would turn blue, owing to the formation of K.HO. In order to find if
the blue colour is not produced by an alkali (ammonia) in the gas, a portion of the papt-r
is not soaked in the potassium iodide, but moistened with water ; this portion will then
also turn blue if ammonia be present. A reagent for distinguishing ozone from hydrogen
peroxide with certainty is not known, and therefore these substances in very small quan-
tities (for instance, in the atmosphere) may easily be confounded.
202 PRINCIPLES OF CHEMISTRY
from it in stability, and by the fact that it oxidises a number of sub-
stances very energetically at the ordinary temperature. In this
respect ozone resembles the oxygen of certain unstable compounds, or
oxygen at the moment of its liberation.
In ordinary oxygen and ozone we see an example of one and the
same substance, in this case an element, appearing in two states. This
indicates that the properties of a substance, and even of an element,
may vary without its composition varying. Very many such cases
are known. Such cases of a chemical transformation which determines
a difference in the properties of one and the same element are termed
isomerism. The cause of isomerism evidently lies deep within the
essence of the nature of a substance, and its investigation has already
led to a number of results of unexpected importance and of immense
scientific significance. It is easy to understand the difference between
substances containing different elements or the same elements in
different proportions. That a difference should exist in the latter
case necessarily follows, if, as our knowledge compels us, we admit
that there is a radical difference in the simple bodies or elements.
But when the quality and quantity of the elements (the composition)
in a substance are the same and yet its properties are different,
then it becomes clear that the conceptions of the elements and of the
composition of compounds, alone, are insufficient for the expression of
all the diversity of the properties of the matter of nature. Something
else, still more profound and internal than the composition of sub-
stances, must, judging from isomerism, determine the properties and
transformation of substances.
On what is the isomerism of ozone with oxygen, and the peculiarities
of ozone, dependent ? In what, besides the store of energy, which in its
way expresses the peculiarities of ozone, resides the causes of its difference
from oxygen 1 These questions for long occupied the minds of investi-
gators, and were the motive for the most varied, exact, and accurate
researches, which were chiefly directed to the study of the volumetric
relations exhibited by ozone. In order to acquaint the reader with the
previous researches of this kind, I cite the following from a memoir by
Soret,in the * Transactions of the French Academy of Sciences ' for 1866 :
4 Our present knowledge of the volumetric relations of ozone may be
expressed at the present time in the following manner :
'1. " Ordinary oxygen in changing into ozone under the action of
electricity shows a diminution in volume." This was discovered by
Andrews and Tait.
' 2. " In acting on ozonised oxygen with potassium iodide and other
substances capable of being oxidised, we destroy the ozone, but the
AND HYDEOGEN PEROXIDE— DALTON'S LAW 203
volume of the gas remains unchanged." Indeed, the researches of
Andrews, Soret, v. Babo, and others showed that the quantity of oxygen
absorbed by the potassium iodide is equal to the original contraction of
the volume of the oxygen — that is, in the absorption of the ozone the
volume of the gas remains unchanged. From this it might be imagined
that ozone, so to say, does not occupy any room — is indefinitely
•dense.
'3. "By the action of heat ozonised oxygen increases in volume,
and is transformed into ordinary oxygen. This increase in volume
corresponds with the quantity of oxygen which is given up to the
potassium iodide in its decomposition " (the same observers).
' 4. These indubitable experimental results lead to the conclusion
that ozone is denser than oxygen, and that ozone in its oxidising
action gives off that portion of its substance which distinguishes it by
its density from ordinary oxygen.'
If we imagine (says Weltzien) that n volumes of ozone consist of n
volumes of oxygen combined with m volumes of the same substance, and
that ozone in oxidising gives up m volumes of oxygen and leaves n
volumes of oxygen gas, then all the above facts can be explained ;
otherwise it must be supposed that ozone is indefinitely dense. ' In
order to determine the density of ozone (we again cite Soret) recourse
cannot be had to the direct determination of the weight of a given
volume of the gas, because ozone cannot be obtained in a pure state.
It is always mixed with a very large quantity of oxygen. It was
necessary, therefore, to have recourse to such substances as would
absorb ozone without absorbing oxygen and without destroying the
ozone. Then the density might be deduced from the decrease of
volume produced in the gas by the action of this solvent in comparison
with the quantity of oxygen given up to potassium iodide. Advantage
must also be taken of the determination of the increase of volume
produced by the action of heat on ozone, if the volume previously
occupied by the ozone before heating be known.' Soret found two such
substances, turpentine and oil of cinnamon. ' Ozone disappears in the
presence of turpentine. This is accompanied by the appearance of a
dense vapour, which fills a vessel of small capacity (0- 14 litre) to such an
extent that it is impenetrable to direct sun-rays. On then leaving the
vessel at rest, it is observed that the cloud of vapour settles ; the
clearing is first remarked at the upper portion of the vessel, and the
brilliant colours of the rainbow are seen on the edge of cloud of
vapour.' Oil of cinnamon — that is, the volatile or odoriferous substance
of the well-known spice, cinnamon — gives under similar circumstances
the same kind of vapours, but they are much less voluminous. On
sure. ,vc.) and making a scries of coi 1 1 j uirat i ve determinations. Sort-t
obtaintMl tli'- t'i .l!n\\ in-- result : two volumes »>t' ozone capable of beiim
dissolved, when df>iroved (1»\- heating a wire to a rc«l heat by a
galvanic current) increase by one volume. Hence it is evident thai in
the formation "t ozone three volumes of oxvu'en u'ive two volumes of
ozone tliai is. it- density (referred to hydrogen ) = _ I .
The observations and determinations of Soret sho\\-ed that ozone is
hea\'ier than oxygen, and e\'en than carbonic anhydride (because
o/.oni>ed oxyu'cn parses from tine orifices more slowly than oxygen
and than its mixtures with carbonic anhydride), although lighter than
chlorine (it flows more rapidly from such orifices than chlorine), and
they al>o indicated that n\nn>' /x on* and " hftff tini'-* d'liy-i' th<in
<>.'->j'/' a . which mav be expressed bv designating a molecule of oxygen
by ( )., and of ozone bv ( ^ \ and which likens ozone to compound sub-
stances'1 formed by oxygen, as. for instance. CO.,, SO,. ()()._,. XO.,, iVc.
This explain- the chief dillerence> between ozone and oxygen, and the
cause of the i>oineri-m. and at the same time leads one to expect "
'hat ozone, as a uas \\'hich is denser than ox\-u'<'ii, would be liquefied
070>7E AND HYDROGEN PEROXIDE— DALTON'S LAW 205
much more easily. This was actually shown to be the case, in 1880, by
Chappuis and Hautefeuille in their researches on the physical properties
of oxygen. Its absolute boiling point is about — 106°, and consequently
compressed and refrigerated ozone when rapidly expanded gives drops,
is liquefied. Liquid and compressed u ozone is blue. In dissolving in
water ozone partly passes into oxygen. Ozone violently explodes when
suddenly compressed and heated, changing into ordinary oxygen, and
evolving, like all explosive substances,12 that heat which distinguishes
it from oxygen.
Thus, judging by what has been said above, ozone should be
formed in nature not only in the many processes of oxidation which
go on, but also by the condensation of atmospheric oxygen. The
significance of ozone in nature has often arrested the attention of
observers. There is a series of ozonometrical observations which show
the different amounts of ozone in the air at different localities, at
different times of the year, and under different circumstances — for
instance, on the appearance of epidemics. But the observations made
in this direction cannot be considered as sufficiently exact, because the
methods in use for determining ozone were not quite accurate. It is
however indisputable 13 that the amount of ozone in the atmosphere is
subject to variation ; that the air of dwellings contains no ozone (it dis-
appears in oxidising organic matter) ; that the air of fields and forests
always contains ozone, or substances (peroxide of hydrogen) which act
like it ; that the amount of ozone increases after storms ; and that
miasms, £c., are destroyed by ozonising the atmosphere. It may be
imagined that the influence exerted by ozone on animal life is due to
the fact that it easily oxidises organic substances, and miasms are
formed of organic substances and the germs of organisms, which are
easily changed and oxidised. Indeed, many miasms — for instance,
conditions, evidently be less capable of passing into a state of gaseous movement, should
sooner attain a liquid state, and have a greater cohesive force.
11 The blue colour proper to ozone may be seen through a tube one metre long con-
taining oxygen 10 p.c. ozonised. The density of liquid ozone has not, as far as I am
aware, been determined.
12 All explosive bodies and mixtures (gunpowder, detonating gas, &c.) evolve heat in
exploding (in giving a greater number of molecules from one molecule, and sometimes
several substances from one substance, as in the explosion of nitro-compounds ; see later) —
that is, the reactions which accompany explosions are exothermal. In this manner
ozone in decomposing evolves latent heat, although generally heat is absorbed in
decomposition. This shows the meaning and cause of explosion.
13 In Paris it has been found that the further from the centre of the town the greater
the amount of ozone in the air. The reason of this is evident : in a city there are many
conditions for the destruction of ozone. This is why we distinguish country air as being
fresh. In spring the air contains more ozone than in autumn ; the air of fields more than
the air of towns.
206 PRINCIPLES OF CHEMISTRY
the volatile substance of decomposing organisms — are clearly destroyed
or changed not only by ozone, but also by many powerfully oxidising
substances, such as chlorine with water, potassium permanganate, and
the like.14
Thus in ozone we see (1) the capacity of elements (and it must
be all the more marked in compounds) of changing in properties with-
out altering in composition ; this is termed isomerism ; 15 (2) the
capacity of elements for arranging themselves in molecules of 'different
densities ; this forms a special case of isomerism called polymerism •
(3) the capacity of oxygen for appearing in a still more intense and
energetic chemical state than that in which it occurs in ordinary
gaseous oxygen ; and (4) the formation of unstable equilibria, or
chemical states, which are expressed both by the ease with which ozone
acts as an oxidiser and in its capacity for decomposing with explo-
sion.16
Hydrogen peroxide. — Many of those properties which we have seen
in ozone belong also to a peculiar substance containing oxygen and
hydrogen, and called hydrogen peroxide, or oxygenated water. This
substance was discovered in 1818 by Thenard. When heated it is
decomposed into water and oxygen, evolving as much oxygen as is
contained in the water remaining after the decomposition. That
portion of oxygen by which hydrogen peroxide differs from water be-
haves in a number of cases just like the active oxygen in ozone, which
distinguishes it from ordinary oxygen. In H2O2, and in O3, one atom
of oxygen acts in a powerfully oxidising manner, and on separating out
14 The oxidising action of ozone may be taken advantage of for technical ends ; for
instance, for destroying colouring matters. It has even been employed for bleaching
tissues and for the rapid preparation of vinegar, although these methods have not yet
received wide application.
15 Isomerism in elements is termed allotropism.
16 A number of substances resemble ozone in one or another of these respects. Thus
cyanogen, C.^N.^, nitrogen chloride, &c., decompose with an explosion and evolution of
heat. Nitrous anhydride, N./)3, forms a blue liquid like ozone, and in a number of cases
oxidises like ozone. Bed phosphorus is to white phosphorus, in a certain sense, what
oxygen is to ozone, and in other respects the reverse ; this is also a case of allotropism.
Thus a chemical analogy is diffused in different and most varied directions, and it is only
after an acquaintance with the diverse relations of substances that an idea can be formed
of the complexity of chemical changes, whilst their general system is still wanting; that
is to say, there is nothing analogous to and explaining the correlation of liquid to
gaseous substances. But there is reason to think that in this case also an explanation
will arise with the accumulation of data, as we see from the fact that the conception of
dissociation explained in the simplest manner a number of chemical relations which
without it were not at all clear. It should be here observed that the transition
between oxygen and ozone under the conditions of a silent discharge forms a reversible
reaction which is subject to the conception of dissociation, whilst, exempt from the
conditions of a silent discharge, the passage of ozone into oxygen is not reversible, and
forms an instance of decomposition in the strictest sense.
o/n NE AND HYDROGEN PEROXIDE — DALTnN'S LAW 207
it leaves H20 or O2, which do not act so sharply, although they still
contain oxygen.17 Both contain the oxygen in a compressed state, so
to speak, and when freed from pressure by the forces (internal) of the
elements in another substance, this oxygen is easily evolved, and there-
fore acts like oxygen at the moment of its liberation. Both substances
in decomposing, with the separation of a portion of their oxygen, evolve
heat, while an absorption of heat is usually required for decomposi-
tion.
Hydrogen peroxide is formed under many circumstances by com-
bustion and oxidation, but in very limited quantities ; thus, for instance,
it is sufficient to shake up zinc with sulphuric acid, or even with water,
to remark the formation of a certain quantity of hydrogen peroxide in
the water.18 From this cause, probably, a series of diverse oxidation
processes are accomplished in nature, and, according to Prof. Schone, of
Moscow, hydrogen peroxide occurs in the atmosphere, although in vari-
able and small quantities, and probably its formation is connected with
ozone, with which it has much in common. The usual case of the
formation of hydrogen peroxide, and the means by which it may be in-
17 It is evident that there is a want of words here for distinguishing oxygen, O, as an
ultimate element, from oxygen, Oo, as & free element. It should be called oxygen gas, did
not habit and the length of the expression render it inconvenient.
18 Schiinbein states that the formation of hydrogen peroxide is to be remarked in every
oxidation in water or in the presence of aqueous vapour. According to Struve, hydrogen
peroxide is contained in snow and in rain-water, arid its formation, together with ozone
and ammonium nitrate, is even probable in the processes of respiration and combustion.
A solution of tin in mercury, or liquid tin amalgam, when shaken up in water containing
sulphuric acid gives rise to the formation of hydrogen peroxide, whilst iron under the
same circumstances does not give rise to its formation. The presence of small quantities
of hydrogen peroxide in these and similar cases is recognised by many reactions
Amongst them, its action on chromic acid in the presence of ether is very characteristic.
Hydrogen peroxide converts the chromic acid into a higher oxide, Cr2O7, which is of a
dark-blue colour, and dissolves in ether. This ethereal solution is to a certain degree
stable, and therefore the presence of hydrogen peroxide may be recognised by mixing
the liquid to be tested with ether and adding several drops of a solution of chromic acid.
On shaking the mixture the ether dissolves the higher oxide of chromium which is
formed, and acquires a blue colour. The formation of hydrogen peroxide in the combus-
tion and oxidation of substances containing or evolving hydrogen must be understood in
the sense of the conception, to be considered later, of molecules occupying equal volumes
in a gaseous state. At the moment of its evolution a molecule H.> combines with a mole-
cule O2 and gives H3O2. As this substance is unstable, a large proportion of it is
decomposed, a small amount only remaining unchanged. If it is obtained, water is easily
formed from it ; this reaction evolves heat, and the reverse action is not very pro-
bable. Direct determinations show that the reaction H2O2 = H2O + O evolves 22000 heat
units. From this it will be understood how easy is the decomposition of hydrogen
peroxide, as well as the fact that a number of substances which are not directly
oxidised by oxygen are oxidised by hydrogen peroxide and by ozone, which also evolves
heat on decomposition. Such a representation of the origin of hydrogen peroxide has
been developed by me since 1870. In recent times Traube has pronounced a similar
opinion.
208 PRINCIPLES OF CHEMISTRY
directly obtained,111 is by the double decomposition of an acid and the
peroxides of certain metals, especially those of potassium, calcium, and
barium.20 Among these peroxides, that of barium is the most
conveniently obtained, it being enough, as we saw when speaking of
oxygen (Chap. III.), to heat the anhydrous oxide of barium to a red heat
in a current of air or oxygen ; or, better still, to heat it with potassium
chlorate, and then to wash away the potassium chloride also formed.21
Barium peroxide gives hydrogen peroxide by the action of acids in the
cold.22 The process of decomposition is very clear in this case ; the
hydrogen of the acid replaces the barium of the peroxide, a barium salt
of the acid being formed, while the hydrogen peroxide formed by the
19 The formation of hydrogen peroxide from barium peroxide by a method of double
decomposition is an instance of a number of indirect methods of prepa/Tafaon. A sub-
stance A does not combine with B, but AB is obtained from AC in its action on HP (see
Introduction) when CD is formed. Water does not combine with oxygen, but as a hydrate
of acids it acts on the compound of oxygen with barium oxide, because this oxide gives a
salt with an acid anhydride ; or, what is the same, hydrogen with oxygen does not directly
form hydrogen peroxide, but when combined with a haloid (for example, chlorine), under
the action of barium peroxide, BaOo, it leads to the formation of a salt of barium and H._,(\>.
It is to be remarked that the passage of barium oxide, BaO, into the peroxide, BaO,>, is
accompanied by the evolution of 121000 heat units per 16 parts of oxygen by weight
combined, and the passage of H.,O into the peroxide H._>O._> does not proceed directly,
because it would be accompanied by the absorption of 22000 units of heat by 10 parts
by weight of oxygen combined. Barium peroxide, in acting 011 an acid, evidently evolves
less heat than the oxide, and it is this difference of heat that is absorbed in the hydrogen
peroxide. Its energy is obtained from the energy evolved in the formation of the salt of
barium.
20 Peroxides of lead and manganese, and other analogous peroxides (see Chapter III.,
Note 9), do not give hydrogen peroxide under these conditions, but yield chlorine
with hydrochloric acid.
21 The impure barium peroxide obtained in this manner may be easily purified. For
this purpose it is dissolved in a dilute solution of nitric acid. There will always remain
a certain quantity of an insoluble residue, from which the solution is separated by filtra-
tion. The solution will contain not only the compound of the barium peroxide, but also
a compound of the barium oxide itself, a certain quantity of which always remains un-
combined with oxygen. The acid compounds of the peroxide and oxide of barium are
easily distinguishable by their stability. The peroxide gives an unstable compound, and
the oxide a stable salt. By adding an aqueous solution of barium oxide to the resultant
solution, the whole of the peroxide contained in the solution may be precipitated as a
pure aqueous compound. The first portions of the precipitate will consist of impurities —
for instance, oxide of iron. The barium peroxide separates out, and is collected on a
filter and washed ; it then forms a substance having an entirely definite composition,
BaOo,8H2O, and is very pure. Pure hydrogen peroxide should always be prepared from
such purified barium peroxide.
22 In the cold, strong sulphuric acid with barium peroxide gives ozone; when diluted
with a certain amount of water it gives oxygen (see Note 6), and hydrogen peroxide is
only obtained by the action of very weak sulphuric acid. The acids hydrochloric,
hydrofluoric, carbonic, and hydrosilicofluoric, and others, when diluted with water also
give hydrogen peroxide with barium peroxide. Professor Scho'ne, who investigated
hydrogen peroxide with great detail, showed that it is formed by the action of many of
the above-mentioned acids on barium peroxide.
0/ONE AND HYDROGEN PEROXIDE — D ALTON'S LA\V 209
barium peroxide remains in solution.23 The reaction is expressed
by the equation BaO2 + H2SO4=H2O2 + BaSO4. It is best to take a
weak cold solution of sulphuric acid and to almost saturate it with
barium peroxide, so that a small excess of acid remains; insoluble
barium sulphate is formed. A more or less dilute aqueous solution
of hydrogen peroxide is obtained. This solution may be concentrated
in a vacuum over sulphuric acid. In this way the water may even be
entirely evaporated from the solution of the hydrogen peroxide ; only
in this case it is necessary to work at a low temperature, and not to
keep the peroxide for long in the rarefied atmosphere, as otherwise it
decomposes.24
When pure, hydrogen peroxide is a colourless liquid, without smell,
and having a very unpleasant taste — such as belongs to the salts of
many metals— the so-called ' metallic ' taste. Water held in zinc vessels
has this taste, which is probably due to its containing hydrogen peroxide.
The tension of the vapour of hydrogen peroxide is less than that of
aqueous vapour ; this enables its solutions to be concentrated in a
vacuum. The specific gravity of anhydrous hydrogen peroxide is 1'455. _^ ,
Pure hydrogen peroxide decomposes, with the evolution of oxygen, when
heated even to 20° (by the action of light ?). But the more dilute its
aqueous solution the more stable it is. Very weak solutions may be
distilled without the hydrogen peroxide decomposing. It decolorises
solutions of litmus and turmeric, and acts in a similar manner on many
colouring matters of organic origin (for which reason it is employed for
bleaching tissues).
Many substances decompose hydrogen peroxide, forming water and
oxygen, without apparently suffering any change. In this case sub-
stances in a state of fine division evince an incomparably quicker action
23 With the majority of acids, that salt of barium which is formed remains in solution ;
thus, for instance, by employing hydrochloric acid, hydrogen peroxide and barium chloride
remain in solution. Complicated processes would be required to obtain pure hydrogen
peroxide from such a solution. It is much more convenient to take advantage of the
action of carbonic anhydride on the pure hydrate of barium peroxide. For this purpose
the hydrate is stirred up in water, and a rapid stream of carbonic anhydride is passed
through the water. Barium carbonate, insoluble in water, is formed, and the hydrogen
peroxide remains in solution, so that it may be separated from the carbonate by filtering
only. On a large scale hydrofluosilicic acid is employed, because its barium salt is also
insoluble in water.
24 Hydrogen peroxide may be extracted from very dilute solutions by means of ether,
which dissolves it, and when mixed with it the hydrogen peroxide may even be distilled.
A solution of hydrogen peroxide in water may be enriched by cooling it to a low tempera-
ture, when the water crystallises out — that is, is converted into ice — whilst the hydrogen
peroxide remains in solution, as it only freezes at very low temperatures. It must be
observed that hydrogen peroxide, in a strong solution in a pure state, is exceedingly
unstable even at the ordinary temperature, and therefore it must be preserved in vessels
always kept cold, as otherwise it evolves oxygen and forms water.
VOL. I. P
•210
than compact masses, from which it is evident that the action is here
li:isp<l on contact i .-•• • Introduction). It is enough to hrinij hydrogen
peroxide into contact \\ith charcoal, e/old, the peroxide of manganese
or lead, the alkalis, metallic silver, ami platinum, to bring about the
above decomposition.*'1 l>e>ide> which, livdro^eii |>ero\ide forms water
and part- with it> oxv^'en \\ith uivat ea-e to a number of substances
which arc capable of being oxidised or of combining \\'ith oxygen, and
in this respect i- very like ozone and ot her /,mr, /_•/'/// ,,.,'i<lis' /-x.'-'1 To
the numlier of contact phenomena, which are so natural to hydrogen
peroxide, as a substance which is unstable and ea.-ilv decomposable with
the evolution of heat, must be referred the following- -that in the pre-
xeiice ot many substances containing oxvu'en it evolves, not only its own
oxvgen. but also that of the substances which are brought into contact
with it that i-. /'/ <'<•(* In <> r<-<l//<-i/ir/ indnm-r. It behaves thus wit h
ozone, the oxide^ of silver, mercury, gold and platinum, and lead
peroxide. The oxvgen in these -ubstances is not stable, and therefore
the feeble inllueiice of contact is enough to destroy its position.
i'i --t a IT] i. ci • rt a in (it tin1 r<i/ttli/t /<' i>r contact jiliriK 'iiu-na
timi. wliiNt. h.. Wfv.-r.it (l..«.sii..t iiltrr tin- S.TH-S of c-Iimip-s
liens ..nly. I'n.f.-sscr Scli.".n.- of tin- 1 '.•! mtT-ky . \i-a.lnny.
i idy cxjilainc.l a nninli.T of i1. -act ions of h\ driven peroxide \vliich prrviou-ly \\ •'!•<•
• ,.1 und'-r-i 1. Tim-, for instance, lie showed iliat witli liydrop-n jicroxidc. alkali^ "'ivc
IH I'M id. - n: the alkaliin' metals, \\hirli cMinliinc \\-ith the remaining hydrc.Li'i'ii pci'Mxiilf.
un-talilc f nil] Mimd- \vhirharc easily decoiii]ioscd. and therefore allcali- evince
,i di-ciiin]in iT.talvtii i Iliience mi - ihilimis of hydrogen |>eroxide. Only acid >«'ln-
• |.ei'Mxide. and tht-n ..nly dilute ones, can !..• pre-erved well.
:"" // '•'//"'-. a- a -nil-:.-, nee conlaiiiin^ nnirh Mxvufen ( namely. Id ]iart> to
ar-eiKc. CM • ' in.- intu cal. ide, the oxides of x.inc and cMjiper into jx-roxides ;
I p.irt- wit en 1 m\ suljiliide-. I-MII\ ei'tin.L; th.'in iiit.. sulphates, iVc. So. lur
, vain],!. . / . iTl I !; cli 1, ad r-lllpllide. I'l.S. into \\hite lead .lllphate. I'I, SO,. CM]. per
• . -I i er ^nlphate. and -o MH. The iv-iMrali.ii i of ..Id ..il pa i nt in--- 1 1\ liydrM-vn
:. I. i- lia-i .1 MII this a li-iii. Oil-, IMHI'S are n-nally admixed with wliite lead, and in
it | true. M| 1 inie. This i- ]>art ly
: .. to tin ilphn elli-d li\.h.i/.n cMiitained in (he air. \\hi.-h acts MM white lead.
i.-ad ulj.hi.l.'. whieh i- Mark. The intermixture .,| the I, lark colMiir darkens the
r. t. In i-I. i tureuitha ..luli..ii <.f h\.lr,,-.-n |.i-r..\i«l.-. tin- lilai-k li-atl stilpliiilr
nf il.. i i il-i i !....• ....,: , . M them. HxdiM-en pn-Mxidr oxidisi-s
with it »•" '1 ' nl.:, MM. . Tim it , mpM.-,,-, h\ dri.. die acid, sett in^ the iodine
fl'. e and CM|,\r-f1 tin I.T ; il I. I'MinpM-es -Illplmretted
in in <•:• art lh< • er. t! ihe ; iilplmr Ire... Starch paste with
, did. t. I i. u. ,r. direr! , ; , (,,. , ,f huln.p.,, in the entire
:d,-ence .,f tree ... id : Lilt I he a dd i t i..| , , ,| ;, s Ilia 1 1 . | Ua 11 t it V of in .)! Illphate I- feel i \ itl'i.ill
,, i- ,,f l..,id aci lal •• J,, 1 hi' m:\tnr. i . : .. .!i . 1 1 I . i r , , i , r, -i •. i ,!,,,-],, .,, t ),,. p;l.,t,.. This i:, a very
,..'.' •.•:•:]• ,• •;• ••• h\drM . . , d-.. the test with i-hroniii- iiriil
i.lirl fill, i • '• N"1 -
OZONE AND HYDROGEN PEROXIDE — D ALTON'S LAW '211
Hydrogen peroxide, especially in a concentrated form, in contact with
these substances, evolves an immense quantity of oxygen, so that an
explosion takes place and an exceedingly powerful evolution of heat is
observed if hydrogen peroxide in a concentrated form be made to fall
in drops upon these substances in dry powder. An exactly similar de-
composition takes place in dilute solutions.27
Just as a whole series of metallic compounds, and especially the
oxides and their hydrates, correspond with water, so also there are
many substances analogous to hydrogen peroxide. Thus, for instance,
calcium peroxide is related to hydrogen peroxide in exactly the same
way as calcium oxide or lime is related to water. In both cases the
hydrogen is replaced by a metal — namely, by calcium. But it is most
important to remark that the nearest approach to the properties of
hydrogen peroxide is afforded by a non-metallic element, chlorine ; its
action on colouring matters, its capacity for oxidising, and for evolving
oxygen from many oxides, is analogous to that exhibited by hydrogen
peroxide. Even the very formation of chlorine is closely analogous to the
formation of peroxide of hydrogen ; chlorine is obtained from manganese
peroxide, MnO2, and hydrochloric acid, HC1, and hydrogen peroxide from
barium peroxide, BaO2, and the same acid. The result in one case is
essentially water, chlorine, and manganese chloride ; and in the other
case there is produced barium chloride and hydrogen peroxide. Hence
water + chlorine corresponds with hydrogen peroxide, and the action
of chlorine in the presence of water is analogous to the action of
hydrogen peroxide. This analogy between chlorine and hydrogen
peroxide is expressed in the conception of an aqueous radicle, which
(Chap. III.) has been already mentioned. This aqueous radicle (or
hydroxyl) is that which is left from water if it be imagined as deprived
of half of its hydrogen. According to this method of expression, caustic
soda will be a compound of sodium with the aqueous radicle, because it
is formed from water with the evolution of half the hydrogen. This is
expressed by the following formulae : water, H2O, caustic soda, NaHO,
27 To explain the phenomenon an hypothesis has been put forward by Brodie, Clausius,
•and Schonbein which supposes ordinary oxygen to be an electrically neutral substance,
composed of, so to speak, two electrically opposite aspects of oxygen — positive and negative.
It is supposed that hydrogen peroxide contains one kind of such polar oxygen, whilst in
the oxides of the above-named metals the oxygen is of opposite polarity. It is supposed
that in the oxides of the metals the oxygen is electro-negative, and in hydrogen
peroxide electro-positive, and that on the mutual contact of these substances ordinary
neutral oxygen is evolved as a consequence of the mutual attraction of the oxygens of
opposite polarity. Brodie admits the polarity of oxygen in combination, but not in an
uncombined state, whilst Schonbein supposes uncombined oxygen to be polar also, con-
sidering ozone as electro-negative oxygen. The supposition of the oxygen of ozone being
other than that of hydrogen peroxide is contradicted by the fact that in acting on barium
peroxide strong sulphuric acid forms ozone, and dilute acid forms hydrogen peroxide.
p 2
212 PRINCIPLES OF CHEMISTRY
just as hydrochloric acid is HC1 and sodium chloride NaCl. Hence the
aqueous radicle HO is a compound radicle, just as chlorine, Cl, is a
simple radicle. They give hydrogen compounds, HHO, water, and HC1,
hydrochloric acid ; sodium compounds, NaHO and NaCl, and a whole
series of analogous compounds. Free chlorine in this sense will be
C1C1, and hydrogen peroxide HOHO, which indeed expresses its
composition, because it contains twice as much oxygen as water.
Thus in ozone and hydrogen peroxide we see examples of very
unstable, easily decomposable (by time, spontaneously, and on contact)
substances, full of the energy necessary for change,28 capable of
being easily reconstructed (in this case decomposing with the evolu-
tion of heat) ; therefore they are examples of unstable chemical
equilibria. If a substance exists, it signifies that it already presents a
certain form of equilibrium between those elements of whicli it is built
up. But chemical, like mechanical, equilibria exhibit different degrees
of stability or solidity.29
28 The lower oxides of nitrogen and chlorine and the higher oxides of manganese
are also formed with the absorption of heat, and therefore, like hydrogen peroxide, act in
a powerfully oxidising manner, and are not formed by the same methods as the majority
of other oxides. It is evident that, being endowed with a richer store of energy (acquired
in combination or absorption of heat), such substances, compared with others poorer
in energy, will exhibit the greatest diversity of cases of chemical action with other sub-
stances.
29 If the point of support of a body lies in a vertical line below the centre of gravity, the
equilibrium is entirely unstable. If the centre of gravity lies below the point of support,
the state of equilibrium is very stable, and a vibration may take place about this posi-
tion of stable equilibrium, as in a pendulum or balance, which ends in the body passing
to its position of stable equilibrium. But if, keeping to the same mechanical example,
the body be supported not on a point, in the geometrical sense of the word, but on a
small plane, then the state of unstable equilibrium may be preserved, unless destroyed
by external influences. Thus a man stands upright supported on the plane, or several
points of the surfaces of his feet, having the centre of gravity above the points of support.
Vibration is then possible, but it is limited, otherwise on passing outside the limit of
possible equilibrium another more stable position is attained about which vibration
becomes more possible. A prism immersed in water may have several more or less
stable positions of equilibrium. It is the same with the atoms in molecules. Some
molecules present a state of more stable equilibrium than others. Hence from this simple
comparison it will be already clear that the stability of molecules may vary considerably,
that one and the same elements, taken in the same number, may give isomerides of different
stability, and, lastly, that there may exist states of equilibria which are so unstable, so
ephemeral, that they will only arise under particularly special conditions — such, for
example, as certain hydrates mentioned in the first chapter (see Notes 57, (57, and others).
And if in one case the instability of a given state of equilibrium is expressed by its
instability with a change of temperature or physical state, then in other cases it is
expressed by the case of decomposition under the influence of contact or of the purely
chemical influence of other substances. However clearly the greater or less stability
of the elementary structure of substances be depicted to us in these general considera-
tions, still at present there is no possibility of presenting them in a sufficiently con-
crete form to enable purely mechanical conceptions to be applied to them; that is,
to subject them to mathematical analysis, and to master the subject to such an extent
OZOM- AND HYUKOGEN PEROXIDE— DALTON'S LAW 213
Besides this, hydrogen peroxide indicates another side of the subject
which is not less important, and is much clearer and more general.
Hydrogen unites with oxygen in two degrees of oxidation : water
or hydrogen oxide, and oxygenated water or hydrogen peroxide ; for a
given quantity of hydrogen the peroxide contains twice as much oxygen
as does water. This is a fresh example confirming the correctness of
the law of multiple proportions, of which we have already made men-
tion in speaking of the water of crystallisation of salts. Now we can
formulate this law with entire clearness — the law of multiple propor-
tions. If two radicles A, and B (either simple or compound substances),
unite together to form several compounds, AnBOT, A^Br . . . ., then
having expressed the compositions of all these compounds in such a ivay
that the quantity (by weight or volume) of one of the component parts
will be a constant quantity A, it will be observed that in all the compounds
AB((, AB,, . ... the quantities of the other component part, B, will
always be in commensurable relation : generally in simple multiple
proportion — that is, that a : b . . ., or m/nis to r/q as whole numbers,
for instance as 2 : 3 or 3 : 4. . . .
The analysis of water shows that in 100 parts by weight it contains
11-112 parts by weight of hydrogen and 88*888 of oxygen, and the
analysis of peroxide of hydrogen shows that it contains 94-112 parts of
oxygen to 5 -888 parts of hydrogen. In this the analysis is expressed,
as analyses generally are, in percentages ; that is, it gives the amounts
of the elements in a hundred parts by weight of the substance. The
direct comparison of the percentage compositions of water and hydrogen
peroxide does not give any simple relation. But such a relation is
immediately observed if we calculate the composition of water and of
hydrogen peroxide, having taken either the quantity of oxygen or the
quantity of hydrogen as a constant quantity — for instance, as unity. The
most simple proportions show that in water there are contained eight
parts of oxygen to one part of hydrogen, and in hydrogen peroxide
sixteen parts of oxygen to one part of hydrogen ; or one-eighth part of
hydrogen in water and one- sixteenth part of hydrogen in hydrogen
peroxide to one part of oxygen. Naturally, the analysis does not give
these figures with absolute exactness — it gives them within a certain
degree of error — but they approximate, as the error diminishes, to that
limit which is here given. The comparison of the quantities of hydrogen
and oxygen in the two substances above named, taking one of the com-
ponents as a constant quantity, gives an example of the application of
as to foretell the degree of stability of different chemical states of equilibrium. The
commencement of elementary generalisations has been apprehended in only a few
214 PRINCIPLES OF CHEMISTRY
the law of multiple proportions, because water contains eight parts and
hydrogen peroxide sixteen parts of oxygen to one part of hydrogen, and
these figures are commensurable and are in simple proportion as 1 : 2.
An exactly similar multiple proportion is observed in the composition
of all other well-investigated definite chemical compounds,30 and there-
fore the law of multiple proportions is accepted in chemistry as the
starting point from which other considerations are judged.
The law of multiple proportions was discovered at the very
beginning of this century by John Dalton, of Manchester, in investigat-
ing the compounds of carbon with hydrogen. It appeared that two
gaseous compounds of these substances — marsh gas, CH4, and olefiant
gas, C2H4, contain for one and the same quantity of hydrogen quanti-
ties of carbon which stand in multiple proportion ; namely, marsh gas
contains relatively half as much carbon as olefiant gas. Although the
analysis of that time was not exact, and did not give Dalton results
in complete accordance with truth, still the accuracy of this law,
recognised by Dalton, was confirmed by further more accurate investiga-
tions. On establishing the law of multiple proportions, Dalton gave a
hypothetical explanation for it. This explanation is based on the
atomic theory of matter. In fact, the law of multiple proportions is
understood with unusual ease by admitting the atomic structure of
matter.
50 When, for example, any element forms several oxides, they are subject to the
law of multiple proportions. For a given quantity of the non-metal or metal the
quantities of oxygen in the different degrees of oxidation will stand as 1 : 2, or as 1 : 3, or
as 2 : 3, or as 2 : 7, and so on. Thus, for instance, copper combines with oxygen in at
least two proportions, forming the oxides found in nature, and called the suboxide and
the oxide of copper, Cu2O and CuO ; the oxide contains twice as much oxygen as the sub-
oxide. Lead also presents two degrees of oxidation, the oxide and peroxide, and in the
latter there is twice as much oxygen as in the former, PbO and PbO.,>. The substance
known under the name of minium, and which is somewhat widely used as a red paint,
is only a mixture of the mutual compounds of these oxides, which is proved not only by
the inconstancy of its composition, but also by the fact that reagents capable of extract-
ing the oxide of lead, especially acids, do actually extract it and leave lead peroxide.
When a base and an acid are capable of forming several kinds of salts, normal, acid, basic,
and anhydro-, it is found that they also clearly exemplify the law of multiple proportions.
This was demonstrated by Wollaston soon after the discovery of the law in question. We
saw in the first chapter that salts show different degrees of combination with water of
crystallisation, and that they obey the law of multiple proportions. And, more than
this, the indefinite chemical compounds existing as solutions may, as we saw in the same
chapter, be brought under the law of multiple proportions by the hypothesis that solu-
tions are unstable hydrates formed according to the law of multiple proportions, but
occurring in a state of dissociation. By means of this hypothesis the law of multiple
proportions becomes still more general, and all the aspects of chemical compounds are
subject to it. The direction of the whole contemporary state of chemistry was deter-
mined by the discoveries of Lavoisier and Dalton. By bringing indefinite compounds
also under the law of multiple proportions we arrive at that unity of chemical conceptions
M/oNK AND HYDROGEN PEROXIDE— DA LT< >.Vs LAW 215
The essence of the atomic theory is that matter is supposed to con-
sist of an agglomeration of small and indivisible parts — atoms — which do
not fill up the whole space occupied by a substance, but stand apart
from each other, as the sun, planets, and stars do not fill up the whole
space of the universe, but are at a distance from each other. The form and
properties of substances are determined by the position of their atoms in
space and by their state of movement, while the phenomena accomplished
by substances are understood as redistributions of the relative positions
of atoms and changes in their movement. The atomic representation of
matter arose in very ancient times,31 and up to recent times was at strife
with the dynamical hypothesis, which considers matter as only a mani-
festation of forces. At the present time, however, the majority of
scientific men uphold the atomic hypothesis, although the present con-
ception of an atom is quite different from that of the ancient
which was impossible so long as definite compounds were separated from indefinite by a
sharp line of demarcation.
51 Leucippus, Democritus, and especially Luoretius, in the classical ages, repre-
sented matter as made up of atoms — that is, of parts incapable of further division. The
geometrical impossibility of such an admission, as well as the conclusions which were
deduced by the ancient atomists from their fundamental propositions, prevented other
philosophers from following them, and the atomic doctrine, like very many others, lived,
without being ratified by fact, in the imaginations of its followers. Between the present
atomic theory and the doctrine of the above-named ancient philosophers there is naturally
a remote historical connection, as between the doctrine of Pythagoras and Copernicus,
but they are essentially profoundly different. For us the atom is indivisible, not in
the geometrical abstract sense, but only in a physical and chemical sense. It would be
better to call the atoms indivisible individuals. The Greek atom = the Latin individual,
according to both the sum and sense of the words, but historically these two words are
endowed with a different meaning. The individual is mechanically and geometrically
divisible, but only indivisible in a definite sense. The earth, the sun, a man or fly
are individuals, although geometrically divisible. Thus the atoms of contemporary
science, indivisible in a physico-chemical sense, form those units which are concerned in
the investigation of the natural phenomena of matter, just as a man is an indivisible unit in
the investigation of social relations, or as the stars, planets, and luminaries serve as units
in astronomy. The formation of the vortex hypothesis, in which, as we shall afterwards
see, atoms are entire whirls mechanically complex, although physico-chemically indivisible,
already shows that the scientific men of our time in holding to the atomic theory have
only borrowed the word and form from the ancient philosophers, and not the essence of
their atomic doctrine. It is erroneous to imagine that the contemporary conceptions of
the atomists are nothing but the repetition of the metaphysical reasonings of the
ancients. As a geometrician in reasoning about curves represents them as formed of a
sum total of straight lines, because such a method enables him to analyse the subject
under investigation, so the scientific man applies the atomic theory as a method
of analysing the phenomena of nature. Naturally there are people now, as in ancient
times, and as there always will be, who apply reality to imagination, and therefore
there are to be found atomists of extreme views ; but it is not in their spirit that we
should acknowledge the great services rendered by the atomic doctrine to all science,
which, while it has been essentially independently developed, is, if it be desired to
reduce all ideas to the doctrines of the ancients, a union of the ancient dynamical and
atomic doctrines.
•2ir>
philo-nphers. Now. an atom i- regarded ratlier a- an isolate or
which i> indivisible by physical :v- and chemical forces, \\-hilst the atom
of the ancients \\as mechanically and uvometricallv indi\ i-iMe. \\ hen
I 'alt on ( 1 v| ' 1 ) discovered t he la w of mult iple propoi t ions, he pronounced
himself in favour of the atomic doctrine, because it enables this law to
Ke very easily understood. If the divisibility of everv element has a
limit, namely the atom, then the atoms ot element- are the extreme
limits of all di visibilit v. and t hey ditl'er from each other in t heir nat ure,
and the tormation ot a compound trom elementary matter must consist
in the a-'uTe-'at ion of several different atoms into one whole or system
of atoms, now termed mi.rti1'?''* or ///'-/<*•///,>•. As atoms can onlv com-
bine in their entire masses, n i- evident that not onlv the law of defi-
nite coi M posit ion. but a 1 so i hat of multiple proport ions, must apply to the
combination of atoms with one another ; for one atom of a substance
can combine with one. two. or three atoms of another substance, or in
iM'iier;i 1 one. t wo. i h ive atoms ( if one siib-t anee are able t < » combine with
one. t wo. or t hive atoms of a not her : this b 'inu' the essence of the law
of multiple proportions. Chemical and physical data are verv well
explained by the aid of the atomic theory. The displacement ot one
element by another follows the law of equivalency. In this case one
or several atom- of a Lnven element take the p'ace < if one or several
a t oms ot another element in its compounds. I he at on is of di lie rent
-ubstances can lie mixed together in the same sense a- sand can be
1 1 1 1 xed v. 1 1 1 1 da v. "I I ie\- do not unite i n' o one \\ hole /.<•.. t here is not a
perfect blendiiiLf in the one or other case, but only a juxtaposition, a
homogeneous whole beinu' formt d from individual parts. This is
the tir-t and mo.-i simple form of applying the atomic theory to the
explanation of chemical phenomena.'1'*
, ini iili-i';iilv clc;irl\ -yiiilinlixcd tin- ilitTci-i'iifc dt' tlu-ir iijiininM fi'inn
- . . . . • .-M,.;. -lit . NM\\ inil\ tin' iii(li\i(lu;ils nt ihi' clcinclits. iiuli-
| ,-!n .... ;nv t.Tliicil iiti.ni-, and 111.' indlN idlliiU nt' emu
. . .
. ••',,. , . - l|lll.l>x,T\illllr. i||\ i-il,l(..illl(l
: lili- tn lllldi'l--t:iui| i-itlitT I i'_'ht en-
|,., , . . ,| I i,,. i nt liM-rllil Ilii-il I. ]ill\ -iriil. ur rllf|llii-;ll
! . . ' ' i-j||i-llt ill ;'nilli;iU nlllv. hilt to IIS till1 ^Illilllr-t
|,|, ... • ',,, ir , •,.•,'•• nii-t inli. 'I'llll- iii"! i- 'li li;i- lici-i Ulic il d i)irc|it mil
, , | ] | In ci,iii-i-|>t i<>ii «i in.it N-I-. .Did 1 111 - lilt-. |iri'|i;irnl t I it • ^rn Hi I id fur t hr
t . . , ] , | ( | ,, . 1 1 -. 1 1 j i i 1 h \ i . ' ' . i 'I llii' i nil -t it lit ii ill nl 1 1 i.i I! i T. Ill thr ;i 1 1 >lli lc 1 1 1 1 'i >1'V
. , m,ivci ' - "i hfiisfiih L.,di«-. \\ Mli n - -mi . pl.mi't-, mid incti-nrH. i-nilucd witli ever-
OZONE AND Jiyi>i;<x;KN I'Ki;< >X I DK — DAI/fuN's LAW '217
A certain number of atoms n of an element A in combining with
several atoms m of another element B give a compound AnBm, each
molecule of which will contain the atoms of the elements A and B in
this ratio, and therefore the compound will present a definite composition,
expressed by the formula AnBm, where A and B are the weights of the
lusting force of motion, forming molecules as the heavenly bodies form systems, like
the solar system, which molecules are only relatively indivisible in the same way as the
planets of the solar system are inseparable, and stable and lasting as the solar system is
lasting. Such a representation, without necessitating the absolute indivisibility of
atoms, expresses all that science can require for an hypothetical representation of the
constitution of matter. In closer proximity to the dynamical hypothesis of the constitu-
tion of matter is the oft-times revived vortex hypothemt. Descartes first endeavoured
to raise it ; Helmholtz and Thomson gave it a fuller and more modern form ; many
scientific men applied it to physics and chemistry. The idea of vortex rings serves
tis the starting point of this hypothesis; these are familiar to all as the rings of
tobacco smoke, and may be artificially obtained by giving a sharp blow to the sides of a
cardboard box having a circular orifice and filled with smoke. Phosphine, as we shall
see later on, when bubbling from water always gives very perfect vortex rings in a still
atmosphere. In such rings it is easy to observe a constant circular motion about their
axes, and to remark the stability the rings possess in their motion of translation. This
unchangeable maps, endued with a rapid internal motion, is likened to the atom. In a
medium deprived of friction, such a ring, as is shown by theoretical considerations of the
subject from a mechanical point of view, would be perpetual and unchangeable. The
rings are capable of grouping together, and combining, being indivisible, remain
indivisible. The vortex hypothesis has been established in our times, but it has not
been fully developed ; its application to chemical phenomena is not clear, although
not impossible ; it does not satisfy a doubt in respect to the nature of the space existing
between the rings (just as it is not clear what exists between atoms, and between the
planets), neither does it tell us what is the nature of the moving substance of the ring,
und therefore for the present it only presents the germ of an hypothetical conception of
the constitution of matter, consequently, I consider that it would be superfluous to
speak of it in greater detail. However, the thoughts of investigators are now (and
naturally will be in the future), as they were in the time of Dalton, often turned to the
question of the limitation of the mechanical division of matter, and the atomists have
searched for an answer in the most diverse spheres of nature. I select one of the
methods tried, which does not in any way refer to chemistry, in order to show how closely
all the provinces of natural science are bound together. Wollaston proposed the inves-
tigation of the atmosphere of the heavenly bodies as a means for confirming the
existence of atoms. If the divisibility of matter be infinite, then air must extend
throughout the entire space of the heavens as it extends all over the earth by its elasticity
and diffusion. If the infinite divisibility of matter be admitted, it is impossible that any
portion of the whole space of the universe can be entirely void of the component parts of
our atmosphere. But if matter be divisible up to a certain limit only — namely, up to the
atom — then there can exist a heavenly body void of an atmosphere ; and if such a body
be discovered, it would serve as an important factor for the acceptation of the validity of
the atomic doctrine. The moon has long been considered as such a luminary, and this
circumstance, especially from its proximity to the earth, has been cited as the best proof
of the validity of the atomic doctrine. This proof is apparently (Poisson) deprived of
some of its force from the possibility of the transformation of the component parts of
our atmosphere into a solid or liquid state at immense heights above the earth's surface,
where the temperature is exceedingly low ; but a series of researches (Poule) has shown
that the temperature of the heavenly space is, comparatively, not so very low, and is
attainable by experimental means, so that at the low existing pressure the liquefaction
218 PRINCIPLES OF CHEMISTRY
atoms and m and n their relative number. If the same elements A and
B, in addition to AMBm. also yield another compound A,.B<;, then by-
expressing the composition of the first compound by AlirBmr (and this
is the same composition as AHBm), and of the second compound by
AruB5n, we have the law of multiple proportions, because for a given
of gases cannot be expected. Therefore the absence of an atmosphere about the moon,
if it were not subject to doubt, would be counted as a forcible proof of the atomic
theory. As a proof of the absence of a lunar atmosphere, it is cited that the moon,
in its independent movement between the stars, when eclipsing a star — that is, when
passing between the eye and the star — does not show any signs of refraction at its
edge ; the image of the star does not alter its position in the heavens on approach-
ing the moon's surface, consequently there is no atmosphere on the moon's surface
capable of refracting the rays of light. Such is the conclusion by which the absence of
a lunar atmosphere is acknowledged. But this conclusion is most feeble, and there are
even facts in exact contradiction to it, by which the existence of a lunar atmosphere
may be proved. The entire surface of the moon is covered with a number of mountains,
having in the majority of cases the conical form natural to volcanoes. The volcanic
character of the lunar mountains was confirmed in October 1866, when a change was
observed in the form of one of them (the crater Linnea). These mountains must be on
the edge of the lunar disc. Seen in profile, they screen one another and interfere with
-making observations on the surface of the moon, so that when looking at the edge of
the lunar disc we are obliged to make our observations not on the moon's surface, but
at the summits of the lunar mountains. These mountains are higher than those on
our earth, and consequently at their summits the lunar atmosphere must be exceed-
ingly rarefied even if it possess an observable density at the surface. Knowing the mass of
the moon to be eighty-two times less than the mass of the earth, we are able to approxi-
mately determine that our atmosphere at the moon's surface would be about twenty-
eight times lighter than it is on the earth, and consequently at the very surface of the
moon the refraction of light by the lunar atmosphere must be very slight, and at the
heights of the lunar mountains it must be imperceptible, and would be lost within the
limits of experimental error. Therefore the absence of refraction of light at the edge of
the moon's disc cannot yet plead in favour of the absence of a lunar atmosphere. There
is even a series of observations obliging us to admit the existence of this atmosphere,
These researches are due to Sir John Herschel. This is what he writes : — ' It has often
been remarked that during the eclipse of a star by the moon there occurs a peculiar
optical illusion; it seems as if the star before disappearing passed over the edge of the
moon and is seen through the lunar disc, sometimes for a rather long period of time. I
myself have observed this phenomenon, and it has been witnessed by perfectly trust-
worthy observers. I ascribe it to optical illusion, but it must be admitted that the star
might have been seen on the lunar disc through some deep ravine on the moon.' Geniller,
in Belgium (1856), following the opinion of Kassine, Eiler, and others, gave an explana-
tion to this phenomenon ; he considers it due to the refraction of light in the valleys of
the lunar mountains which occur on the edge of the lunar disc. In fact, although
these valleys do not probably present the form of straight ravines, yet it may sometimes,
happen that the light of a star is so refracted that its image might be seen, notwith-
standing the absence of a direct path for the light-rays. He then goes on to remark
that the density of the lunar atmosphere must be variable in different parts, owing to
the very long nights on the moon. On the dark, or non-illuminated, portion, owing to
these long nights, which last thirteen of our days and nights, there must be excessive cold,
and hence a denser atmosphere, while, on the contrary, at the illuminated portion the
atmosphere must be much more rarefied. This variation in the temperature of the
different parts of the moon's surface explains also the absence of clouds, notwithstanding
the possible presence of air and aqueous vapour, on the visible portion of the moon. The
OZONE AND HYDROGEN PEROXIDE— DALTON'S LAW 219
quantity of the first element, A,.,,, there occur quantities of the second
element bearing the same ratio to each other as mr is to qn ; and as //>,
r, q, and n are whole numbers, therefore their products are also whole
numbers, and this is also expressed by the law of multiple proportions.
Consequently the atomic theory is in accordance with and evokes the
first laws of definite chemical compounds : the law of definite composi-
tion and the law of multiple proportions.
So, also, is the relation of the atomic theory to the third law of definite
chemical compounds, the law of reciprocal combining weights, which is as
follows : — If a certain weight of a substance C combine with a weight
ft of a substance A, and with a weight b of a substance B, then, also, the
substances A and B will combine together in quantities a and b (or in
multiples of them). This should be the case from the conception of atoms.
Let A, B, and C be the weights of the atoms of the three substances, and
for simplicity of reasoning let combination proceed in the quantity of one
atom. It is evident that if the substance gives AC and BC, then the
substances A and B will give a compound AB, or their multiple, A,tBTO.
Sulphur combines with hydrogen and with oxygen. Sulphuretted
hydrogen contains thirty-two parts by weight of sulphur to two parts
by weight of hydrogen, which is expressed by the formula H2S. Sulphur
dioxide, SO2, contains thirty-two parts of sulphur and thirty-two parts of
oxygen, and therefore we conclude, from the law of combining weights,
that oxygen and hydrogen will combine in the proportion of two parts
of hydrogen and thirty -two parts of oxygen, or multiple numbers of
them. And we have seen this to be the case. Hydrogen peroxide
contains thirty-two parts of oxygen, and water sixteen parts, to two
parts of hydrogen ; and so it is in all other cases. This consequence of
the atomic theory is in accordance with nature, with the results of
analysis, and is one of the most important laws of chemistry. It is a law,
because it indicates the relation between the weights of substances enter-
ing into chemical combination. Further it is an eminently exact law,
and not an approximate one. The law of combining weights is a law
of nature, and by no means an hypothesis, for let the entire theory of
atoms be cast down, still the laws of multiple proportions and of com-
bining weights will remain, inasmuch as they deal with facts. They
may be guessed at from the sense of the atomic theory, and historically
presence of an atmosphere round the sun and planets, judging from astronomical observa-
tions, may be considered as fully proved. On Jupiter and Mars there may be even
distinguished bands of clouds. Thus the atomic doctrine, admitting a finite mechanical
divisibility only, must be, as yet at least, only accepted as a means, similar to that means
which a mathematician employs when he breaks up a continuous curvilinear line into a
number of straight lines. There is a simplicity of representation in atoms, but there is
,110 absolute necessity to have recourse to them. The conception of the individuality of
the parts of matter exhibited in chemical elements only is necessary and^trustworthy.
220 PRINCIPLES OF CHEMISTRY
the law of combining weights is intimately connected with this theory ;
but they are not identical, but only connected, with it. The law of
combining weights is formulated with great ease, and is an immediate
consequence of the atomic theory, without it, it is even difficult to under-
stand. Data for its evolution existed previously, but it was not seen
until those data were interpreted by the atomic theory. Such is the
property of hypotheses. They are indispensable to science ; they bestow
an order and simplicity which are difficultly attainable without their
aid. The whole history of science is a proof of this. And therefore
one may boldly say that it is better to hold to an hypothesis which may
afterwards prove untrue than to have none at all. Hypotheses facilitate
scientific work and render it uniform. The search for truth, like the
plough of the husbandman, helps forward the Avork of the labourer,
regulates it, and forces him to think of the further improvement both
of the work itself and of its implements.
221
CHAPTER V
NITROGEN AND AIR
GASEOUS nitrogen forms about four-fifths (by volume) of the atmo-
sphere ; consequently the air contains an exceedingly large mass of it.
Whilst entering in so considerable a quantity into the composition of
air, nitrogen does not seem to play any active part in the atmosphere,
the chemical action of which is mainly dependent on the oxygen it con-
tains. But this is not an entirely correct idea, because animal life
cannot exist in pure oxygen, in which animals pass into an abnormal
state and die ; and the nitrogen of the air, although slowly, forms
diverse compounds, many of which play a most important part in
nature, especially in the life of organisms. However, neither plants
nor animals directly absorb the nitrogen of the air, but take it up
from already prepared nitrogenous compounds ; further, plants are
nourished by the nitrogenous substances contained in the soil and water,
and animals by the nitrogenous substances contained in plants and in
other animals. Atmospheric electricity is capable of aiding the passage
of gaseous nitrogen into nitrogenous compounds, as we shall afterwards
see, and the resultant substances are carried to the soil by rain, where
they serve for the nourishment of plants. Plentiful harvests, fine
crops of hay, vigorous growth of trees — other conditions being equal —
are only obtained when the soil contains ready prepared nitrogenous
compounds, consisting either of those which occur in air and water, or
of the residues of the decomposition of other plants or animals (as
in manure). The nitrogenous substances contained in animals have
their origin in those substances which are formed in plants. Thus
the nitrogen of the atmosphere is the origin of all the nitrogenous
substances occurring in animals and plants, although not directly so,
but after first combining with the other elements of air.
The nitrogenous compounds which enter into the composition of
plants and animals are of primary importance ; no vegetable or animal
cell — that is, the elementary form of organism — exists without con-
taining a nitrogenous substance ; organic life, before all, evinces itself in
\ he-e nitrogenous substances. Tin- germs, seeds, and those parts by
which cells multiply themselves abound in nitrogenous substances; the
--tun total of the phenomena \\hidi are proper to organisms depend,
before all. on the chemical properties of the nitrogenous substances
which enter into their eoinposit ion. It is enough, tor instance, t o point
out the fact that vegetable and animal organisms, dearly distinguish-
able as such, are characterised by a dilierent degree of energy in their
nature, and at the same time by a difference in the amount of nitro-
genous substances they contain. In plants, which compared with
animals possess but little activity, being incapable of independent move-
ment, iVc.. the amount of nitrogenous substances is very much less than
in animals, who-e tissues are almost exclusively formed of nitrogenous
substances. It is remarkable that the nitrogenous parts of plants,
chietlv of the lower orders, sometimes present both iorms and properties
which approach to those of animal organisms : tor example, the xoo-
spores of seaweeds, or those parts by means of which the latter multiply
themselves. These xoospores on leaving the seaweed in many respects
re-einble the lower orders of animal life, having, like the latter, the pro-
perty of moving. They also approach the animal kingdom in their com-
jio.-ition, their outer coat containing nitrogenous matter. IMrectlv the
xoospore becomes covered with that non-nitrogenous or cellular coating
which is proper to all the o'-dinarv cells of plants, it loses all re-
semblance to an animal organism and becomes a small plant. 1 1 may be
thought from this that the cause of the diH'erence in the vital processes
of animals and plants is the different amount of nitrogenous substances
'hey contain. Tho-e nitrogenous elements which occur in plants and
animals apperta in to the series of exceedingly coin] 'lex and very change-
able chemical compounds: their elementary composition alone shows
this; besides nitrogen, they contain carbon, hydrogen, oxygen, and
-ulphur. I'eing distinguished by a very great instability under many
condition-; in uhidi other compounds remain unchanged, these sub-
-tance.- are titled for those perpetual changes which form the first con-
dition of \ii;il activity. These complex and changeable nitrogenous
, lances of the or^ani-m are called jir<>t<'nl unhxtn uri-n. The white
(if egg- is a familiar example of such a substance. They are also
contained in the lle-h of animals, the curdy dements of milk, the
glutinous matter of \\heaten Hour, or so called gluten, \\hidi forms the
diicf component < if macan >ni. ive.
Ni'r'>Lfen occur- in the earth crust, in compounds either forming
the remain- of pl:i ni - ;i i id a nimals, or derived t rom the n 1 1 rogen ot the
atmosphere as a con-djuence of it.- combination with the other com-
ponent pan- of the air. It \-^ not found in other forms in the earths
NITROGEN* AND AIR 223
crust ; so that nitrogen must be considered, in contradistinction to
oxygen, as an element which is purely superficial, and does not extend
to the depths of the earth.1
Nitrogen is liberated in a free state in the decomposition of the
nitrogenous organic substances entering into the composition of
organisms — for instance, on their combustion. All organic substances
burn when heated to redness with oxygen (or substances readily yielding
it, such as oxide of copper) ; the oxygen combines with the carbon,
sulphur, and hydrogen, and the nitrogen is evolved in a free state,
because at a high temperature it does not form any stable compound,
but remains free. Carbonic anhydride and water are formed from the
carbon and hydrogen respectively, and therefore to obtain pure
nitrogen it is necessary to remove the carbonic anhydride from the
gaseous products obtained. This may be done very easily by the action
of alkalis— 4»r instance, caustic soda. The amount of nitrogen in
organic substances is determined by a method founded on this.
It is also very easy to obtain nitrogen from air, because oxygen
combines with many substances. Either phosphorus or metallic copper
are usually employed for removing the oxygen from air, but, naturally,
a number of other substances may also be used. If a small saucer 011
which a piece of phosphorus is laid be placed on a cork floating on water,
and the phosphorus be lighted, and the whole be covered with a glass
bell jar, then the air under the jar will be deprived of its oxygen, and
nitrogen only will remain, owing to which, on cooling the water will
rise to a certain extent in the bell jar. The same object (procuring
nitrogen from air) is attained much more conveniently and perfectly
when air is passed through a red-hot tube containing copper filings.
At a red heat, metallic copper combines with oxygen and gives a black-
powder of copper oxide. If the layer of copper be sufficiently long and
the current of air slow, all the oxygen of the air will be absorbed, and
nitrogen alone will pass from the tube.2
1 The reason why there are no other nitrogenous substances within the earth's mass
beyond those which have come there with the remains of organisms, and from the air
with rain-water, must be looked for in two circumstances. In the first place, in the in-
stability of many nitrogenous compounds, which are liable to break up with the forma-
tion of gaseous nitrogen ; and in the second place in the fact that the salts of nitric acid,
forming the product of the action of air on many nitrogenous and especially organic
compounds, are very soluble in water, and on penetrating into the depths of the earth
(with water) give up their oxygen. The result of the changes of the nitrogenous organic
substances which fall into the earth is without doubt frequently, if not always, the forma-
tion of gaseous nitrogen. Thus the gas evolved from coal always contains much nitrogen
(together with marsh gas, carbonic anhydride, and other gases).
2 Copper (best as shavings, which present a large surface) absorbs oxygen, forming
CuO, at the ordinary temperature in the presence of solutions of acids, or, better still, in
224 PRINCIPLES OF CHEMISTRY
Nitrogen may also be procured from many of its compounds ivitk
oxygen* and hydrogen^ but the best fitted for this purpose is a saline
mixture containing, on the one hand, a compound of nitrogen with
oxygen, termed nitrous anhydride, N2O3, and on the other hand,
ammonia, NH3 — that is, a compound of nitrogen with hydrogen. By
heating such a mixture the oxygen of the nitrous anhydride combines
with the hydrogen of the ammonia, forming water, and gaseous nitrogen
is evolved, 2NH.< + N203 = 3H2O -f- N4. Nitrogen is procured by
this method in the following manner : — A solution of caustic potash is
saturated with nitrous anhydride, by which means potassium nitrite is
formed. On the other hand, a solution of hydrochloric acid saturated
with ammonia is prepared ; a saline substance called sal-ammoniac,
NH4C1, is thus formed in the solution. The two solutions thus pre-
pared are mixed together and heated. Reaction takes place according
to the equation KNO, + NH4C1 == KC1 + 2H2O -f N2. This reaction
proceeds in virtue of the fact that potassium nitrite and ammonium
chloride are salts which, on interchanging their metals, give potassium
chloride and ammonium nitrite, NH4NO2, which breaks up into water
and nitrogen. This reaction does not take place without the aid of
heat, but it proceeds very easily at a moderate temperature. Of the
resultant substances, the nitrogen only is gaseous, the potassium chloride
is non-volatile, and is left behind in the vessel in which the solutions
are heated. Pure nitrogen may be obtained by drying the resulting
gas and passing it through a solution of sulphuric acid (to absorb, a
certain quantity of ammonia which is evolved in the reaction).
Nitrogen is a gaseous substance which does not much differ in
physical properties from air ; its density, referred to hydrogen, is
approximately equal to 14 — that is, it is slightly lighter than air ; one
litre of nitrogen weighs 1-256 grams. Nitrogen mixed with oxygen,
the presence of a solution of ammonia, when it forms a bluish-violet solution of oxide
of copper in ammonia. Nitrogen is very easily procured by this method. A flask
is filled with copper shavings and closed with a cork furnished with a funnel and stop-
cock. A solution of ammonia is poured into the funnel, and caused to slowly drop upon
the copper. If at the same time a current of air be slowly passed through the flask
(from a gasholder), then all the oxygen will be absorbed from it and the nitn^ni
will pass from the flask. It should be washed with water to retain any ammonia that
may be carried off with it.
3 The oxygen compounds of nitrogen (for example, NoO, NO, NO2) are decomposed
at a red heat by themselves, and under the action of red-hot copper, sodium. A.V., they
give up their oxygen to the metals, leaving the nitrogen free. According to Meyer and
Langer (1885), nitrous oxide, N2O, decomposes below 900°, although not completely, whilst
the decomposition of nitric oxide, NO, does not start at 1200°, but is complete at 1700°.
4 Chlorine and bromine (in excess), as well as bleaching powder (hypochlorites). take
up the hydrogen from ammonia, NH5, leaving nitrogen. Nitrogen is best procured from
ammonia by the action of a solution of sodium hypobromite on solid sal-ammoniac.
NITIMMIKX AND A IK 225
which is slightly heavier than air, forms air. It is a gas which, like
oxygen and hydrogen, is difficultly liquefied, and but little soluble in
water and other liquids. Its absolute boiling point5 is about — 140°;
above this temperature it is not liquefiable by pressure, and at lower
temperatures it remains a gas at a pressure of 50 atmospheres. Liquid
nitrogen boils at — 193°, so that it may be employed as a source of great
cold. At about —203°, in vaporising under a decrease of pressure,
nitrogen solidifies into a colourless snow-like mass. Nitrogen does not
burn, does not support combustion, is not absorbed by any of the re-
agents used in gas analysis, at least at the ordinary temperature — in a
word, it presents a whole series of negative chemical properties ; this is
expressed by saying that this element has no energy for combination.
Although it is capable of forming compounds both with oxygen and
hydrogen as well as with carbon, yet these compounds are only formed
under particular circumstances, to which we will directly turn our atten-
tion. At a red heat nitrogen combines with boron, titanium, and silicon,
forming very stable nitrogenous compounds,6 whose properties are
entirely different from those of nitrogen with hydrogen, oxygen, and
carbon. However, the combination of nitrogen with carbon, although
it does not take place directly between the elements at a red heat, yet
proceeds with comparative ease by heating a mixture of charcoal with
an alkaline carbonate, especially potassium carbonate or barium carbo-
nate, to redness, carbo-nitrides or cyanides of the metals being formed ;
for. instance, K2CO3 + 4C + N, = 2KCN + 3CO.7
Nitrogen is found with oxygen in the air, but they do not readily
combine. Cavendish, however, in the last century, showed that nitrogen
combines with oxygen under the influence of a series of electric sf>arks.
Electric sparks in passing through a moist8 mixture of nitrogen and
oxygen — for instance, through air — cause these elements to combine,
5 See Chapter II. note 29.
6 The combination of boron with nitrogen is accompanied by the evolution of suffi-
cient heat to raise the mass to redness; titanium combines so easily with nitrogen that it
is difficult to obtain it free from that element. It is a remarkable and instructive fact
that the compounds of nitrogen with these non-volatile elements are very stable, and
are themselves non-volatile. Probably in this case the physical state of the substance
with which the nitrogen combines, and ,the state in which the nitrogenous substance is
obtained, evinces its influence. Thus carbon (C = 12) with nitrogen gives cyanogen, C;>N2,
which is gaseous and very unstable, and whose molecule is not large, whilst boron (B = ll)
forms a nitrogenous compound which is solid, non- volatile, and very stable. Its compo-
sition, BN, is essentially like that of cyanogen, but its molecular weight is probably
greater.
7 This reaction, as far as is known, does not proceed beyond a certain limit, probably
because cyanogen, CN, itself breaks up into carbon and nitrogen.
8 Fremy and Becquerel took dry air, and observed the formation of brown vapours of
oxides of nitrogen on the passage of sparks.
VOL. I. Q
funning reddish-lirowii fumes of oxides of nit ro^vn.'' which form with
\\ ater a co]n| on i n 1 ci i] it am 1 1 iu' nit ro;_;e n, oxygen, and hydrogen namely,
nitric acid.1" N 1 1 < ' ... The presence of t lie lat ter is easily reco^iii.M'd, not
only ir.'iii it-- I'IM Menu iu' litmus paper, Itut al>o Irom its acting as a
l"'\verful oxidistT even <if inci'cuvv. Conditions similai1 to these occur
111 nature, during a thunderstonn or in otlier eleetneal discharges
accomplished in the atmosphere 
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